Explodemon

You will get this while going for the trophy Bring it down! Bring it down!. However you can get it at any time against some of the bigger enemies. You will see a number appear over your normal counter indicator - 2, 3, trophy unlocks. Just keep pushing and it will pop. It is not difficult at all and you will probably get it without actually going for it, so don't worry too much if you don't get it right away after unlocking the ability.