You re way too early in the game and have barely any champions. Focus on saving blue shards and popping them during x2 events to get your rooster expanded.

Long term plan?

Getting your starter (galek) in decent lifesteal gear is enough to beat brutal campaign on his own.

Frozen banshee is one of the best poisoners for CB, so focus on building her up eventually. For now focus on having her as fast as possible and having accuracy thats slightly higher than x10 the level of clan boss. Ideally you should have her with high defense and high HP cant hurt either; ignore her offensive stats completely.

Royal guard is amazing nuker both for final bosses (Spider / Dragon) and for waves. Look to build him up when you can with near 100% crit rate and as much crit damage you can.

Warmaiden is decent support that helps your carries deal way more damage (by applying decrease DEF debuff). You should build her so she has accuracy high enough to land that debuff and if you can afford it you can build her with some crit rate and dmg too. Make sure she s slightly slower than your carries, so she first puts Decrease DEF and then your carries hit with their AoE abilities.

Good luck with the grind.

Ok! So, currently, I've got my FB in 5* accuracy, immortal, and defense gear. Who should I build first?

I mean, who should I rank up first?

Frozen Banshee is a good candidate to get 6* next. But make sure she has sufficient accuracy. As a rule of thumb dungeon level x 10. Maybe one accuracy set is not enough to achieve that. She is great at Clan Boss and Dragon and helps you to get better gear for your champs that way.

In general your roster is to small right now. You don't have a good healer, what is quite important for your progression in dungeons. The enemies, not only the boss, but the waves as well, hit back in higher dungeon levels. You definetly need some healing and defense. The only champ that has some kind of healing mechanic is Achak, but his heals are more like a nice addition to your team. He will not make a champ survive that is heavily injured with only a few % hp left. An Apothecary would fit great to your account, or Rector Drath, or Scyl, just waite 6 months 😺 and you get her fro free as log-in reward.

From the champs you allready own, F. banshee, Samar, Royal Guard and Warmaiden are worth 6*. You can skip Warmaiden if you find a better champ for AoE def decreasing (Stag Knight, Dhukk, Armina and some others), but Warmaiden is easy to book. You can farm copies of here in campaign and use them, so Warmaiden will have a 100% debuff-chance easily, while all the epics that can do the same job better than her are harder to max out.

And do yourself a big favour and forget about 3* the campaign right now. 4 champs worth lvl 60 allready, 12-3 brutal is your place to be. You can do the campaign 3* mission easily, when you have a bunch of lvl 60 champs and you can switch your 2-champ-team according to the affinities and enemie moves in the actual stage. Wasting energy and time to try that now (to early) will not do anything for your progression.