Yeah, the boss fight in the extreme stage doesn't mess around. Kerria was taking ~10k damage per hit even with -65% pest attack power and the damage reduction from Taunt, although the hits were thankfully rare due to her evasion.
Nutaku and DMM IGN: Tomitain
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Nutaku Flower Knight Girl ID: 610834201 (inactive)
DMM FKG ID: 106750967
DMM FKG Allies: 109/123
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DMM Girls Symphony ID: 34644951 (inactive)
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Nutaku Mist Train Girls ID: 3355988
DMM MTG ID: MSTL0Q45K5E9
MTG Allies: 31/50 (Nutaku), 11/50 (DMM)
The entire gimmick of those maps is "use skills or die."