Level 10 on the fourth floor... That seems like EXP is coming a bit too fast. Level 10 contains late-game perks.
Hm. Well, a whole bunch of perks need changing anyway, and it occurs to me that maybe the damage needs to ramp up faster than it currently does, if you're able to take that thing down while constantly getting hit.
Something to try, if you would, just because I'd like to see what happens: Go into Labyrinth's test chamber, and fight it there, and see how that goes.