Smash all those who oppose you with your patented smash-hammer. The more you smash, the bigger it'll grow. Eventually, you'll be smashing everyone and everything.
Not only does smashing other players increase the size of your hammer, but so does smashing the gravestones around the map. That drops XP to pick up.
Master the flick-smash. It's easy to catch people unaware if you stop, flick around, and smash them before they can react.