The Gunk

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.

Another easy way to get this done is when you are climbing the central shaft/elevator in you will fight one of the larger beasts on your 2nd stop due to there being gunk. If you let it charge at you near the edge it will fall off and instantly die also unlocking the achievement.

Pretty self explanatory but if you're having issues with it, one thing to note is the little monsters start to pulse with a red circle when they are about to jump/charge at you.. So just stand as close to the edge of a cliff or near the gunk rivers edge in any of the critter encounters and let them jump at you. If you fall off the cliff or into the river don't worry, you'll basically respawn right away and one of them might even jump at you while you're falling/sinking = achievement unlocked.