I can't beat DT scarab due to lack of shields, so crafting even 3*-4* untouchable sets is not on the table at the moment. I do have enough for a full immunity set, though it's not great, but that leaves her with too low defense/HP to survive nukes from the kind of teams I face. Swapping it for an untouchable at some point would instead leave her vulnerable to disables, so it might fail for that reason and I'm back to having to build resist up anyway. Regardless, I don't think I really have the nuking power to really take advantage of this strategy. Even with Seeker and Madam Serris, I'm struggling to one-shot anyone, let alone an entire team.
Anyway, to answer the question about the rest of the roster:
Arena team: Seeker (lead, 217 speed), Madame Serris, Athel, Spirithost (in that turn order). Occasionally I replace Spirithost with High Khatun lead if I think I can win the speed race, but since that's just a dice toss I tend to avoid speed teams. My AoE options are limited, I'm even considering investing in a Kael I found in a mystery shard a couple of days ago. The purpose of Spirithost is just to make the enemy waste their debuffs in their first round, but replacing her with another nuker might give me enough damage to take out the other team outright in some cases.
CB team: Seeker, Apothecary, Jareg, Athel, Frozen Banshee. It's really just about keepin Frozen Banshee alive placing poison for as long as possible. Athel is just there because I got nothing better and she can do a bit of damage with her 3-hit giant slayer procs. I can 2-key brutal with a bit of luck (or if I do it manually), but often I need 3 if my team AI decides to make bad choices.
Dungeon team: Athel, Madame Serris, Apothecary are my core. Doompriest, Shaman, Frozen Banshee and Seeker I swap in occasionally. Mostly I keep Shaman in there for safety in case I lose a champion due to some unfortunate crit procs or something, and my reviver options are limited (Cardinal and Mistress of Hymns are the others). Frozen Banshee I use on dragon mostly, Doompriest on ice golemn and minotaur to deal with the constant debuffs.
Fire knight team: Under development. Athel, Apothecary and Coldheart are given. I originally leveled up Coffin Smasher to 50 for this as well, but I don't know if that's a good long-term solution. I'm thinking just adding Seeker to let the other guys hit more (and he got a 2-hit A1 himself), and some form of support as the last one, be it Shaman or Doompriest.