LOVING the game, but hells teeth - it's so difficult?

I don't even think I'm that far into it. I'm up to the boss? guy on a ledge in a room with a pit in the middle, and he sends out at least 3 waves of mooks to attack me, one with 3, one with 4, and the third wave with 5. I Just can't survive the third wave, no idea if there's even more after that.

Does anyone have any hints/tips other than "git gud"? Where did you put your opening skills, which weapons to use, etc?