I've made a pretty good strategy for the Arena. My record with it is ~41 million.
You need :
Tumbleweed (Forced for this Tournament)
Threepeater
Torchwood (If you don't have him, it's fine)
Appease-mint (If you place him and Tuskmaster freezes him in ice, his effect will work forever)
Double Grapeshot (Cherry Bomb, Bombegranate, or Jalapeño are fine, but Grapeshot w/ Imitater does truckloads of damage)
Typical Score is ~25 million, ~2.5 million if done w/out Appease-mint glitch.
Enjoy!