To keep it simple, the achievement can only be done with the base Imp, since none of the other mechs can stomp. All you have to do is raise the flag of power in the backyard and have plants surround you and whipping your mech. Stomp the ground on the plants, but don't eject yet. Let your stomp recharge and get surrounded by plants again. Stomp, eject, let it recharge, repeat till you get the achievement.