I need to defeat my friend.

So my bud has been going on for nearly 4 years about how undefeated he is in warhammer. Constantly plays head to heads and spouts off about how he has never lost. I need to knock him down a peg. The hard part is he is actually good at the game, but I've seen sometimes he messes up. He mains the beast men though he is skilled at a lot of the factions. What faction should I play that's the most likely to take him out and why?