This is for beating Episode 2 (apart from chapter 7, which is just dialogue). At the end of chapter 6, you will have to fight a good number (I think anywhere from 12-15) of striders and each strider comes with 1 or 2 hunters. Not too difficult if it's on easy. The invincibility cheat also helps if you want to use it. It won't disable achievements. It is LB, Up, RB, Up, LB, LB, Up, RB, RB, Up. The refill ammo cheat also helps out a lot to refill your rocket ammo. For this, you need to switch the gravity gun to the back button instead of the default right bumper. You can do this in the controller options. The cheat is Y,B,A,X,RB,Y,X,A,B,RB. Once you defeat all of the striders and hunters, the achievement will unlock.