It's static, not HP% based and it will always hit your units fucking hard. I remember not being able to perfect that stage during the 1st release coz 2 of my squads died trying to kill that ass. The only time where the boss deals HP% based damage is the very 1st event (there could be others but this is the one I really remember), our 1st yandere girl Lavender.

Fun fact: Lavender is a "stalker" yandere type, different from Convallaria.