This can be done at your very first encounter with "Gunk Critters" and you'll likely get this by playing the game normally anyway. But when you've got a Gunk Critter attacking you, every so often he'll make a lunge/jump at you, try to align yourself so that when he lunges he lands off a ledge (cliff). Although it mentions a "Cliff" this achievement seems to pop even if he's lower than his starting height before the lunge. As you'll see in my video below.