[@Rai]
How about rpg heart system? like 3 hearts = 6 hits (just an example), normal atack, takes 1 hit(half a heart), strong atk takes 2 hits(1 heart), "ultimate" atk (takes 1,5 hearts). It's simple, it revolves around being hit a decent amount of times to be beaten up/close to death. IF you mostly want for our chars to live the story but not necesarily DIE, you can always use the "you are unconscious/in coma for a while and will have a penalty to x/y/z for x period".

just a thought random idea related to the HP topic, unless you already have your own ideas :)