Ben is the main player you control throughout the game that is constantly changing into the other alien forms. Just defeat 50 enemies in your untransformed human state and this will unlock. This is probably best done at the start of the game to get it out of the way. At the end of level 1 you need to jump over some retracting spikes and enter an arena of sorts with the mid boss. Instead of killing the boss just keep killing his minions with Ben until the achievement unlocks. Even if you die, you restart right there and dying does not reset your total kills so keep at it. If you killed all the other enemies you could with Ben from the beginning of level 1, then you’ll get the achievement by replaying this fight within a couple of minutes.