My player's Berzerker is simply overpowered, hitting at 2d10+27 pen 9 on every turn with his power fist, obliterating everything in one turn, and I find balancing to be difficult as making hard hitting enemies would cause a TPK, and making ungodly tough enemies would make it difficult for the rest of the party to kill things. He says he finds it boring too that he kills everything in one turn, but I'm not sure how to balance it out without unbalancing the group.

Any help?

The first mistake was even allowing him to do and/or acquire the things that get him to those numbers. You got steamrolled by a player than wants to power game.

Rule #2 of GM'ing: Say no more than you say yes.