awesome game, took a few tries to figure out the enemies and what they do. Going to attempt to beat Master now.
I think that once enemies once killed should not respawn, because this greatly limits the ability to "run away". you can't leave the room you are in, if not you get swamped by enemies that respawn in the previous rooms.
There are early times when i save the fire/explosion/poison barrels in the other rooms for just in case when i have to run back to use them, but i get swamped by respawned enemies when i backtrack too far.