After unlocking Vader, I also found GW to be frustratingly difficult. I beat it maybe a handful of times until I hit level 85, at which point I began catching up. One thing I will ask first is, what level are you, and are you already facing zeta squads and such?
Since I have hit level 85, I have been having consistent success with GW using only a handful of squads. At one point I completed almost 40 in a row without getting stumped, which I felt was pretty good considering the luck I had earlier in the game. I will note here, however, that at this point I only encounter the occasional zeta squad, but I have seen some of my guildies getting hopelessly screwed against teams with 5 fully maxed, zeta'ed characters that there is no hope of beating. But yeah, with that in mind...
The squads that I have used are as follows:
Rebel Squad: Wedge Lead, Biggs, Lando, Chirrut, StHan/Leia (as needed for extra defense or extra attack)
- Chirrut is a major game changer here, before I had him my rebel team was terrible. His attack is potent at a high level, however, and his debuff cleanse and attack special are both battle-changing also. Wedge and Biggs are both high-powered crit machines, as is Lando, and with STHan's taunt you can totally destroy enemy teams before they can even touch any of your other guys.
Jedi Squad: Lumi Lead, Consular, JKA (sometimes lead if needed for dmg), Ahsoka, Mace.
- Recently just got QGJ as my first zeta, he really changes Jedi squads, but I still found success before with my Lumi lead Jedi Squad against the filler battles
Jawa Squad: Chief Nebit lead, Dathcha, Jawa, Jawa Scavenger, Jawa Engineer
-This all-jawa squad is not super sexy compared to rebels, sith, etc, but the bomb mechanics are devastating and their speed, coupled with Engineer's unique (TM generation and crit chance up), means you can pretty much kill at least 1, if not 2 of their guys before they have a chance to attack.
Sith Squad: DN Lead, EP, Vader, Maul, Dooku
-This squad is perfect when you want to debuff the kitten out of an enemy. EP and dooku deliver stun, Maul has daze, Vader has damage over time and ability block, DN with cooldown extension and annihilate. Very potent squad, on a technical basis they are my "weakest" of these top 4 squads, yet I tend to use them most frequently due to their devastating effects with each other.
Imperial Trooper Squad: Veers lead, Snowtrooper, Magmatrooper, Death Trooper, Shoretrooper
- this is my just for fun squad currently, but due to their synergy they can really wreak some havoc too.
Between these 5 squads, I am able to beat GWs 90% of the time, if not more. For most of this duration that I have been finding consistent success, also bear in mind that a large portion of my characters have not been level 85, or properly modded. I simply do my best to strategize, using my weaker squads first and saving my jawas and rebels for the harder battles. The extra HP and protection can really save you in some of those situations. Also, making sure you aren't playing into the enemy's hands is important. If you're going up against a rebel squad with your empire boys, Biggs is gonna have a field day on you. Hopefully this helps, if not you may be among the ranks of the hopelessly screwed like some of my guild mates. If that is the case, I hope they fix GWs soon for all our sakes