Nintendo Wii Marvel: Ultimate Alliance

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. Where the demons spawn, they will instantly respawn after every death. 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). 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.

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. Thor's Mjolnir's Might or Dare Devil's ES seem perfect for this and only get better as you quickly level. 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. There is no need to worry about mana expensive, the blue orbs start popping so quickly that soon all of the orbs are just adding to the XP.

After a very short period of time, you will start seeing full-momentum orbs start to pop out too, which means that you can start using your Extreme attacks on a regular basis. Rising 16 levels in less than a 30 minutes is probably very doable.