When you enter Mephisto's Realm in Act 2, there's a place where you can get virtually infinite XP and that too in quick succession. This can only be done on the first visit where you have to do certain tasks to light the candles. Go to the one where you have to throw those gargoyle like creatures into the fire, HOWEVER don't actually do that. Instead head towards the edge and start throwing them over the edge (With Wolverine this can be done by holding onto them followed by pressing the Charge button). Near the top, you will encounter a spawn point, where the Gargoyles will keep spawning once you've taken care of the previous gargoyle. As soon as they appear hold onto them, and throw them over (if it misses just quickly go and kill them then return to the spot).

Unlockables

Hard Mode: Beat the game on Normal
Nick Fury: Beat the game on Easy or Normal
Daredevil: Collect the five Daredevil action figures. You should have Daredevil half way through act three.

1. free the guy trapped in a clam (Atlantis). He gives you a code to unlock a close by chest near a fallen submarine

2. Dr. Strange's library

3. Beginning of the circus level in Dooms Castle, break down the door to a storage room

4. Near the Odin statue's left hand in the Bifrost level is a button that will reveal a chest with the action figure

5. Take the right path in the Asgard level. It's on a small path before you get to the houses.
Black Panther: Collect the five Black Panther action figures
Dr. Strange: Save Dr. Strange in Mandarin's Palace
Ghost Rider: Trade places with Ghost Rider in Hell in order to save his soul
Blade: Beat toy grab minigame in the Carnival in Murderworld.
Second & third costumes for all characters: Must play as that character and gain between 2-4 levels.
Fourth costume for all characters: Beat the Combat Training challenge for the character.
Silver Surfer: Defeat Galactus on the Skrull Throneworld.