The Basics:
First of all, you need to utilize your ENTIRE roster, and the more characters you have, the better. There have been several times my strong teams died without taking out anyone in node 12, I don’t think there’s a chance, but I succeed after throwing my 80th guy at them. Now, for this to be effective, you can’t ignore anyone. Yes, this means diverting attention away from your strongest teams for a little while. Whether you are level 80, or just starting GW, getting every character you can and leveling them up at least some is crucial. (If you want a little more incentive, this will also help tremendously in things like territory war, or when surprise events occur which ask for units you don’t have in your elite few.)
----------
Note for beginners and not maxed out players:
At first, you can likely get away with just using a main team, maybe needing a second or third strong team of backups on occasion. Just keep in mind that I have met more high level players that can't beat the Galactic War than those who can, and those who can't typically ONLY train a few core teams, and practically ignore any characters that don't directly help with getting their super meta team. Some modern meta teams can in fact auto fight through GW and win consistently, but it takes a LOT of credits to get them and toons needed to obtain the metas to suitable strengths, and GW is the best *consistent* way to get those funds. ;)
----------
I am NOT saying you need to pay money. If you want to, go ahead. If not, don’t worry about the dozen or so characters that are out of reach. On the bright side, you may still luck into getting some by using the data packs you can purchase using crystals. And pay only characters will eventually become available through other means. Just get who you can at the moment, and you’ll be fine.
Set a minimum level and gear level for all your characters based on what you consider to be the lowest they can be to be in some way useful. I myself set level 53 and gear 4 as the standard. This give the added bonus that all of their abilities are unlocked, and I can give them mods right away if I so chose. Be sure to get all the abilities to level 3 as well, since the first two levels don’t need purple materials (it’s so easy to get green and blue material that it’s only logical to do this). This also is beneficial for championships and territory war for the same reason it helps in GW, even if you take a defeat, throw in a slew of your weaker teams to finish off the last couple in that team so you can continue and potentially get more wins.
Once you have everyone at a minimum level you’re happy with, you do want to start focusing on developing specific teams. Specifically choose teams that are needed for unique events, especially ones that give out large payloads of credits or materials (though I think thats most events now). Don’t worry about making these into your elite teams, just make them good enough to get the job done for their events. This will not only give you functioning stronger teams to fall back on, but focusing on the events will get you more resources to build the next team faster. There are plenty of guides on the best teams and in what order you should harvest characters, but the only one I will parrot here is the phoenix team, as its characters are so versatile in adding them to new teams and they can be used for so many events.
Even among those you don’t train much above your minimum, you should allot most of your characters into at least tentative teams that work well together. This way, even teams of just minimum levels can do something worthwhile. For most of the game it's fine to have some character overlap in your teams, but make sure none of your teams set up for GW have any shared chracters, it wouldn’t do any good if the key member of one team died in the attack of the previous team. While designating your teams, leave out a few of your absolute weakest that won’t help much in teams to use as sacrificial pawns (more on that later).
Just Unlocked Galactic War:
Just a quick note for beginners. If you have already played through it a few times, you can skip this part.
You should always play Galactic War as far as you can, even if you KNOW you can't beat it right now. Use it as a way to earn some extra credits and start collecting shards for the Galactic War exclusive characters (Nightsister Initiate, Cad Bane, Captain Phasma, etc). While you want to use your best team(s) to do the other places, the Galactic War is an excellent place to try out new team combinations.
I would suggest you still follow everything I say in "The Basics", but you can be a bit more lenient with your minimum requirements for your characters. You should still HAVE a minimum level, but level 30, 20, maybe even 10 would be sufficient to begin with. The biggest thing is you never want to send in someone who literally has no chance of doing anything in the fight.
One method you can do to stay balanced is set a low minimum level (5 or 10). Once you get everyone to your minimum, switch to strengthening your designated teams for a bit (the challenge teams and/or your strongest team(s)). Test out how they all do in the Galactic War for a weak or two, and then raise the minimum level a little. Continue this process until all your teams are able to do at least a little damage. All the while, continue to try out new team combinations as their abilities unlock and you discover you like "this" character teamed up with "these" characters.
On that note, perhaps the easiest team choices are those of the same set (droids, ewoks, nightsister, empire, etc...), but especially when starting out, you may not have full teams like that. This is why I suggest getting a large quantity of characters early rather than just a small number of high quality characters. Once you HAVE all the characters you can, that's when you should start building them up. Especially with Mods, it's hard to follow a guide for the "best teams". You can still do that, but Galactic War and high credit reward events are going to be your best sources of income and shards (I have every character in the Galactic War rewards at 7*). With a good income of shards, credits, and training droids, it will be MUCH easier to beef up your best team(s).
My guide is geared a little towards players who have a slightly stronger set of main teams. The general strategy will still work for beginners, but I'll add short notes every so often on how to adjust the strategy for while you are still building yourself up. (Shorter than this one.)
Actually Facing the Galactic War:
You should build up a single elite endurance team which can survive all the way to the end with the aid of weaker teams. You want at least 2 healers, and stunning for any character is ideal. You want characters that can survive a few hits, but can also dish out enough that you can potentially take out one or two enemies before they can even attack. An ideal leader is one that gives you extra speed or extra turns, or else one that boosts your health or defense. I used the Nightsisters with Ventres leading, and before Ventress was able to heal her teammates I sometimes switching the Acolyte for Luminara sacrificing speed and -buff removal for a superb 3rd source of healing. And again, the phoenix team is also great pick.
When fighting, take out as many as you can before they have a chance to attack, later this may mean just one if any. Stun or remove turn meter from taunters first, then those that have assist attacks or else just cause extreme damage like Ray or Leah. If you can’t stun the healers, anyone you can’t kill, make sure they are not much lower than 50% health, thus the healer won’t undo all your work. Sidious and others that give anti-healing buffs are ideal targets to take out first, followed closely by the attackers (who ideally have not been able to attack yet thanks to your stunning). Whenever possible, leave the tanks and healers for last.
----------
An option instead of having stunners or in addition to them is to have de-buffers and ability blockers. Use the same tactics, blocking the healers while de-buffing the taunters when they taunt. In some cases, de-buffers can be better than stunners when it comes to avoiding taunts, because they typically have a better chance of de-buffing than a stunner does of stunning. Also with a de-buffer, the taunt is used up for a few turns and you can focus more on the other enemies, whereas you would need to keep stunning them to avoid the taunt. If you have a mix of stunners and de-buffers, I typically like to stun the heavy hitting attackers while de-buffing the taunters, but judge bassed on the situation.
----------
At the end of each battle (when you have only tanks and healers left), only use basic attacks (this is where having characters that stun with their basic attack are most useful). Heal your team, and LET the enemy healers heal each other, intentionally draw out the battle. You won’t be able to restore your protection in most cases, but this allows you to be at roughly full health and have all your attacks ready at the beginning of each node (and heals would be at least close if not also ready). You should jump for joy when you see Barriss on an enemy team (except for node 12, that usually brings a groan due to how beefed up she and the rest of that team is).
A couple things to consider when starting a fight are retaliators like Duku or Fives, as well as passive abilities such as Daka with her 10% chance to keep an ally from dying. These can potentially cause major setbacks. In some cases it may be a good idea to prioritize them more than anyone else, in others it would be best to leave them for later or even last. These are where you really need to consider the specific strength of the specific enemy, the enemy team as a whole, and your team’s capabilities. When attacking retaliators early, make sure you keep your team well healed through the attack, you don’t want a stray critical hit to take out one of your key characters. Stunners are really nice for taking out retaliators at the end, because with a little luck, you can take them out with little to no damage. Don’t be afraid to sacrifice a special attack or two at the end of a fight if it means the overall survival of your team with good health.
----------
If one of your characters are being taken out far too quickly near the start, try changing the positioning of your team. For example, if Ray or IG-86 1hKOs your weakest character, reset and switch that character with your beefiest character. If you do the EXACT same attack combination, many times the enemy will attack the same SPOT, not the same character. This is not a flawless system, especially if it involves your leader, and may have been patched more recently than I have tested, but I was able to avoid losses in this way countless times when my teams were still in development and did not have the best of health or defence.
----------
The First Six:
If your ONLY strong team is this endurance team, then the first six nodes should be fought with JUST this team. If they can’t survive by themselves, you need to either level them up some more, or change it up a bit (be sure not to forget restarting nodes that you made a mistake on or think could have gone much better). You should also be able to take out nodes 7, 8, and 10 with this team without any real trouble.
However, if you have any other teams that are strong enough to take out the entire node with minimal damage and still have their special attacks and abilities ready at the start of the next battle, then use these teams once each in the first four or five nodes. (If you only have one such team, use them for one easy node while using your main endurance team for the rest.) This will give these teams turn meter, which will be immensely helpful in node 12. Try not to use these teams until node 12, or at least until node 11.
----------
Note for Beginners:
As you develop and beef up your endurance team (which should be one of or consist of members of your best teams), you may not be able to sweep through these nodes. If you simply can't take out a node without losing one of your team members, follow what I say for node 9 for whatever node you are having trouble with.
----------
Node 9:
You should have been able to reach this without loosing any characters (though perhaps with resets to find an optimal combination of attacks). This is the first real challenge for your battle weary team. You likely have no protection left, and the enemy may be stronger than you even at full strength. Try taking them out anyways, being cautious to not let them wipe you out. If you’re lucky, you may be able to get through still without losing anyone. Otherwise, restart and send in another team.
Based on how badly you were almost beaten, determine how strong your “suicide team” should be, and their style. If you just need to get past the initial onslaught, a weak team that can remove the protection of the biggest threat is all you need. Though if it is a particularly bad opposing team, send out a stronger team that can potentially take out one or two of the enemy. Good traits in these are either an attack team with lots of fast heavy hitters, or area effect attackers that can hurt everyone regardless of the situation (especially good against Teebo leaders and lots of taunters). A team I like using for this is my cloan team, the best is when Cody's at the cusp of taking his turn so he can have the whole team zerg rush a particular enemy.
There are a lot more gimmicky and unique characters now than when I first made this guide, such as Bastila Shan or unmasked Kylo Ren with their bonus protection abilities. I would first look up how to beat that specific type of team and see if their is a better kill order for that. If not, or if the counter needed just isn't up to snuff for the particular situation (like a key character for the counter team is not at all trained up), this kamikaze method is my go-to for removing the problem character.
After your suicide team dies, the enemy often has a lot of high turn meters. This is where you use your weakest characters. Select one, and send it out alone. It will die, but now that enemy has no turn meter. Do this as many times as is needed for an optimal setup (one-three). I like using the Gamorrean, for even though I have paid practically no attention to it, it is so tanky it can often take two hits before dying. Otherwise I use characters like the who are simply unfortunate enough to only have shards available in the same place as other characters I want much more.
Nodes 11 and 12:
Node 10 should be no problem, a breath of fresh air in fact. Node 11 can potentially be lethal, while node 12 most certainly will be. These are what you’ve been saving up for. Try taking on node 11 with your endurance team. If you lose one or two members of your team but take out the node, you are probably set. Otherwise, use your judgment on whether you want to sacrifice your endurance team to make it easier for your B team to finish it, or to repeat what you did for node 9, sending in another weaker suicide team.
Node 12 is where you stop worrying about survivability, every team is expendable for the good of the cause. Considering that your weaker teams may be needed, your first targets here are the healers, especially ones that passively heal or constantly grant additional protection. You need to use your strongest teams first to do this. Your best choice for a first team is either an attack heavy team that can take out the biggest threats quickly, or else an area effect team that can remove a lot of protection followed by a FAST attack heavy team. Don’t forget to gauge if you need to sacrifice a single week unit to adjust the turn meters between battles if needed. Don’t worry about getting everyone to low turn meters, for that is usually impossible, someone will hit you earlier than not on your following battle.
Once the healers are gone, your weaker teams should be able to handle everyone else individually. That said, the next largest threats are self healers. Ahsoka should be removed fairly early if possible. Taunters also become a problem unless you have several de-buffers or stunners. Retaliators can be annoying, but they are usually best left for last (if you successfully removed an enemy but only have enough health for one more strike from each of your surviving characters, this is an ideal time to whittle away at a retaliator).
If you can’t eliminate the healers and self healers before you run out of strong teams, throw your specialized weak teams in first. Teams that have good attackers which aren’t fully developed can still do decent damage to any character. Stunners, ability blockers, anti-heals, use every gimmick available to get a few hits in where you can. You want your week teams to focus on one enemy at a time, preferably ones that don’t self heal or heal when you attack. Save a couple decent teams (not exactly weak, but not among your strongest) for finishing off an enemy once it’s been ground down by your other teams. This is especially important when there are still enemy healers, or for facing enemies like cwChewy who have decent self heals that only kick in when they have less health, or Royal guard who will obnoxiously taunt when their ally drops below half health.
After The Run:
The first few times will be rocky as you develop not only your main team, but EVERY team manufactured from your roster. Don’t fret about winning every single time, especially at first. Even if you only finish ten or eleven nodes, that will still get you several training droids and 400k-500k credits. Use your funds to bolster your various teams so that each team can support your efforts. Once you are satisfied with the bare minimum of each team, return to focusing on your best team(s), and simply continue to develop your weaker teams little by little as you go. For example, once all my characters became level 53, I bumped it up to 55, then to 60 (which I am still working on).
----------
Note mostly for Beginners:
Two tidbits about the structure of the Galactic War.
1- You must manually reset the Galactic War before your characters are able to fight again each day, whether you won all twelve nodes or not.
2- The Galactic War will allow up to two days of Galactic War runs at a time: The one you most recently were doing, and the current day's (usually this is "yesterday's" and "today's"). If you do not reset before the next day, you do not get a third run, you simply miss out on a days worth of Galactic War.
Don't worry about completing the Galactic War every day, especially when starting out. When all your characters are used up, simply reset when you can so you can get the next days worth of rewards. But the fact that the Galactic War doesn't automatically reset each day means that if you don't have TIME to finish the Galactic War one day, you can finish it up the following before resetting. Don't get too carried away with this though, for it's unbelievable how easy it is to lose a days run from simply not resetting on time.
----------
After 50:
Once you have beaten GW 50 times, you can sim it! Congratz on getting a free 15+ character shards, 600k+ credits, and lots of other resources each day!
And like I said near the beginning, you now have a base to continue strengthening your arsenal. I sim through the GW every day, and I still unlock every character and get them to level 53 even if I don't forsee using them a lot, because you never know when an event will roll around where you'll need to just throw them in. Since Territory wars came out, I am also slowly working on getting everyone to 6000gp, but that is a side project while I focus on gearing up my main teams for championships and events.
I hope this guide was helpful!