Kill a Slime Abomination with the wrong weapon.
Slime Abomination is these big ghost that rushes you and vomits at you and leaves a big trail of slime behind them. You'll first encounter one on the 3rd level. Be careful when facing them, because they only need one hit to down you. Too get this you need to kill a Slime Abomination with the wrong weapon, for example if the Slime Abomination is yellow you have to kill it with the Blue or Red laser. You can attack it with the right weapon at first, as they do more damage to him, and then finish him of with the wrong weapon when he is low on health.