The name escapes me, but there's a place, in the beginning of the game, where you will come upon a boulder that sits on a ledge. To the left of that ledge is three or four enemies.
By the time you cleared out the place, neither the boulder nor those monsters will be there. Instead, however, in their place will be three jelly-like enemies waiting in hiding. You need to get them to spot you. You and the Majin should fight them. You won't be able to kill them, so they'll be there until you command the Majin to roar. One or two of those creatures will try to siphon the Majin's energy behind his back. Go ahead and hit one of them and then hit the other one if he's also causing pain to the Majin. They will try to strike at you, so just evade their attacks until they focus again on the Majin.
This place is perfect to grind for your Majin Knight achievement, as well as the one before it. Have fun and good luck!
PS: I should also mention that it needs to be nighttime before those creatures can appear. They won't appear in daylight, but in total darkness instead. So wait a good fifteen minutes (real-life time, not the game time) until darkness settles in.