Alternatively, you could throw the demons over the edge (*not* into the lava in the center, or the portal will open and the mission is over) but it isn't very quick. Thor's Mjolnir's Might or Dare Devil's ES seem perfect for this and only get better as you quickly level. If your heroes are capable of quickly killing (one or two attacks) these low HP mobs, you will rapidly level all of your heroes or get costumes of heroes you don't already have. The demons respawn so quickly, that throwing attacks that return (Thor's hammer throw, DD's Extensive Strike for example) can actually kill one on the way out and one on the way back. This is the fastest respawn rate that I've run across in the game (much faster than the Omega Base simulation missions' infinite spawn room or the Asgard simulation area).
Where the demons spawn, they will instantly respawn after every death. In Mephisto's Realm in the beginning area with the alter that reads, 'Thy way shall be lit by the flames of impure souls.' you can garner huge amounts of XP (infinite, if you wish) in a very short amount of time.
