How did you beat S tier dungeon? There simply isn't enough HP to absorb damage from the last boss.
How to beat S tier dungeon (edit and put it as title so people can search this)
get skill that refill HP when Ira get Horny first
Waves 1-3 ez, just get horny and spam atk boost + double attack repeatly
Waves 4 Horny state neutralize at the end of first turn. get almost horny then spam cheering until Ira's HP arround 200 then get her horny (it will refresh her HP) after that spam atk boost + double attack repeatly
Waves 5 just spam atk boost + defense boost.
if u still loosing then just get new upgrade.