Defeat all the enemies in a single round using only rocks to obtain this trophy. This can be done in many rounds, but can easily be done in the very first round alongside  4 Enemies, see that trophy on how to obtain this. Regardless of how you do it, you'll want to make sure the last two enemies get hit by the same rock, because if you don't then the last one will try to escape and be much harder to defeat with a rock.