As it says, you need to kill 100 enemies using the Elephant Stomp. What helps is that this attack is has a larger area of effect than just what you hit, it also hits nearby enemies!
In the Lilac Fields is a good spot to do this one, since you can position yourself where a couple of Scarecrows will spawn infinitely and walk in range of your Stomp. Just constantly spam jump and slam over and over until you get it.