RPG's and FFies have traditionally had three versions of the type of damage I think the OP is looking for.

% of current HP:
Attacks that take a % of your current HP. (thus cannot kill you).

% of Max HP;
Attacks that take a % of your Max HP. (thus, does equal damage however low your HP is and can be lethal)

Critical HP;
attacks that reduce HP to a particular level i.e., "critical" (perhaps about 15% of Max HP) or to "1 HP".


I see what the OP means. It might be nice to have some of the bosses attacks like that, as a better chance to see what your healer is made of.