This achievement can be unlocked as soon as you learn the ability to stop time with your farts. Once you have that ability you can get this achievement by killing all of the enemy's except one. Take the last enemy down to very low health and then activate the ability on his turn, now run up to him and hit him as many times as you can and as long as you kill him/her the achievement will be yours. Please remember that you have to keep the new kid alive so he can use his ability and the fart can only be activated on your opponents turn.
If you would like a video guide for this I will leave mine below.