The Tank Masters is a special tournament for Dragon Quest Heroes: Rocket Slime. After you rescue Morrie Morrie from Callmigh Bluff. You can then talk to Morrie Morrie in the western part of town, and then choose what rank, and what battle to play.

Rank C - Platplanzer, Trailerblazer, Purrsecutor.
Rank B - Carrot Top, Golemater, S. Cargo.
Rank A - Pyramaniac, T-Rax, Enforker.
Rank S - Hooly.