I was playing this game with some help from CoSMOS for the last 2-days.
Sometimes I'm killed in one hit; happens in the Graveyard usually, but the monsters there are brutal.
I'm pretty sure it only happens when the hit does more damage than what you set the maximum health to.
Played a game with health set to 512, and managed to die. Played a game with my health set to 760 (don't know why I picked that number... just decided randomly); and killed the "final boss" (I guarantee you'll know when you're there) without dying.
Curses will kill you instantly, so never open the cursed chest.
As I thought.
Played through again with my health set to 768 (since it's a nice binary number).
Was able to finish without dying.