Floor 309, 12 energy per turn, kills everything super fast. Very addictive build due to assassin blade's damage going up dramatically after improving it with 10+ Golden Whetstones. Enemy HP in endless scales linearly, so doing this lets you outscale them permanently. Most battles are over in a matter of seconds.
Golden whetstones can be farmed by drinking star potions. If you have enough HP you can get tons by letting that blood-sucking bat NPC bite you repeatedly. That's kinda hot