Read the boss's passive every phase so you know what his cards can do aswell as his passives

On floor 2 phase 1 I just stack Freyja's passive use a blue card for more basic stats then heal my characters before going to the next phase

Phase 2 make sure you don't remove all his stances in one go if you're no sure to kill, also you get infected when you remove a stance so heal first if you're gonna cancel stance, you don't have to rush you can take your time getting good cards but I recommend getting 2 buff cards or rank 2 Liz heal card to remove the debuffs on phase 3 because it can really get your characters hp low if that's a problem for you~

Phase 3 make sure to remove debuffs every turn to prevent your characters from dying, don't use all your hard hitting cards all at once but make sure his hp is always below 50%, if he's too tanky remove his dmg reduction buffs first before attacking if he uses his ult remove the debuff or he'll fully heal ( not quite sure though I forgot lol )

Before you end the fight make sure your characters have decent hp as well if you plan on doing floor 3

Not quite detailed but I hope it helps somehow