The first time you will be able to play as ReepiCheep is in the area "Miraz's Castle Assault", which is in the fourth level. Reepicheep is the little mouse-keteer character.
Just switch to Reepicheep and remain in the courtyard in the main hub area. Three Telmarine soldiers will continuously spawn from the door in the back right area of the courtyard. Just keep killing them with Reepicheep's completely awesome mouse-sized rapier until the achievement pops.