You really, really want to complete the final node, but any credits at all help, and I don't think it even lets you attempt the final tier until you reach a certain level (75, maybe? It's been so long I don't remember).
IIRC correctly, the final node requires 5 BH with at least 5* each. If you have a single BH at g11, that toon can solo it and the others are just there to help you qualify. Credit Heist is an old event and wasn't meant for all the power-creep. If you go the solo route, though, you have to go all-in on Tenacity mods, because several of the enemies can stun.
If you can get a g9-g10 Bossk and one other g10-g11 bounty hunter, then Bossk can take lead, boost Tenacity, and then load up on health mods and Bossk can get stunned over and over while the other toon takes everyone out.
Remember, though, all scoundrels are eligible for Credit Heist. BH are great because they are scoundrels but also qualify for BH only events, but since you asked about the Credit Heist specifically, I have to tell you that Raid Han is eligible and does a bang-up job on offense. Bossk Lead (5*/g9) plus Boba (g*/g9) plus Raid Han (5*/g9) is probably overkill, though you'll still need two more scoundrels at 5*/g1/level 1 to qualify.
To get RH shards, all you need is a single 7* toon, doesn't matter what it is, and then join a guild that's doing heroic Rancor. There are lots of low-level guilds who need your 600 energy a day and don't care that you don't personally contribute any damage to the Rancor raid. It's actually best if you can get in a guild that sims Rancor. If you do, you'll have Raid Han at 7* in 5-6 weeks. RH is a wonderful toon that's useful all the way to endgame content, so don't be afraid that any gear you put on him will ever be wasted.
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As for Jango and Hound's Tooth, yeah, you can finish Level 8 missions with a borrowed toon even if you're level 56, but they don't actually let you attempt them until you're much higher level. I know you can't go for him now (I might have even said that earlier) but it's definitely a node you want to start farming as soon as it becomes available. It takes a long time, so the earlier you start the better.
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As a general note, succeeding in Fleet Arena is easier than succeeding in Squad Arena. My character growth improved a lot and my squad arena performance improved faster than it had been (still slow, though) once I went all in on ships. If you can take top5 in fleet, that's 300+ crystals every day that you weren't getting before. To get the same number of crystals in Squad, you have to be top10 and it's MUCH harder in quad to do that.
Whether you're playing for fun or with specific achievement goals in mind or whether you're playing competitively, it's good to get a decent squad Arena team together, but it's even better to get a strong Fleet Arena squad together. The meta in Fleet changes much more slowly, and often (though not always) the characters you need are more accessible (Chewbacca's not easy, but also not as difficult as Malak or GAS). It seems like a big investment, but once you're at the top of fleet it becomes not only easier to stay there, but you have all these extra crystals that make it easier to farm your Squad Arena team or your fun characters or the characters you want for that specific goal you have your eye on.
As I've said before, it's a game, so if you think fleet just s**ks (some people do) then skip it. But you should at least try it out, because if you don't hate it, it can make you much more successful in all your other in-game goals. Some of my favorite parts of the game are in the Dark Side Geonosis Territory Battle (I love the Acklay fight - I'd won like 6 in a row, but then I lost yesterday b/c that's a fight that can still surprise you without ever seeming overwhelmingly difficult or unfair). But I honestly wouldn't have had the time and resources to gear up my geonosians with all my other priorities (like getting GAS) if I hadn't had the 400 crystals I get every day from taking #1 in fleet. So it's a bit of a chore on some days, but it's also fun to know that I have the best fleet on my shard (I really do), and it enables me to do fun things that I wouldn't have been able to do without my success there.
