To enrage a monster, you must nail it square in the head with an object. The monster will then get an angry look on its face. You then know it is enraged. You may get this while completing the game, but if you don't, it is no big deal. This can easily be boosted in about 5 minutes by playing Yellow Jello Level 4 over and over. Enrage the first enemy and kill him with the same grenade. Then blow up the stone wall. Then enrage the second enemy and kill him. 2 enraged kills in about 10 seconds. Keep reloading until it unlocks.