In terms of level 3 dungeons:
I break down all the mechanics of the bosses in there. Give those a read, and if youre having issues just PM me. Mostly there is just "tricks" to each dungeon you can work with.
Like for passing fire 3+, you just want a different affinity (Ice, Lit, whatever) DPS, and Enna (I usually replace a dps with enna). If you dont have enna you need something with a cleanse or Debuff immunity to block the fire stacks - or you have to go heavy with shields (Adolphus, Ice MC) and someone with Attack down on command before she ultis (Sigrid) so you survive the ulti.
The problem is your best shielder and best attack down character on command is sigrid and adolphus, who are fire/poison, so enna is your best bet.

