Pretty easy. Before starting a level (after completing the tutorial), talk to the Mutator character. Turn on the mutator for Infinite Ammo, and perhaps No Cowards so you don't have to chase guys after you start shooting them. Start a level with the Soldier, probably an easy level like Slums. Switch to Machine Gun. Go around shooting everybody. If you kill at least 20 characters on a floor, the Cannibal will unlock and achievement will pop when you use the exit elevator (after completing any required tasks). If it doesn't pop, there may not have been 20 characters. Just try again on the next level.