You get this at the end of the game after defeating Dr. Doom on any difficulty. As with most end of act boss battles, this one comes in stages...
First he'll throw the dark fantastic four at you. This is basically a slugfest that will be over more quickly if you fire off all your super abilities (right trigger-Y) while contained in a small area.
Then Doom will rise up in the middle of the room. He'll be shielded by rays and untouchable. Just for to the end of each of the green rays (the big red X marks the exact spots). It will be easiest if you can manage to step into the alcove of each machine and just start pounding as quickly as you can with your strongest character. This is because you will be blocking a member of the dark F.F. from getting out of the machine. If you are fast enough you can kill them before they get out. If they DO get out, you'll have to chase them around while you beat on them.
After dealing with each of the machines and the Dark F.F. again, go to the center of the room and face off with Doom. He'll grab you and an animated sequence will start where you have to press/mash the correct buttons to get free. This steals some of Doom's Odin-power to make him weaker so you can finally attack him directly. If you miss the button sequence, you'll start it all over again from Doom floating in the middle of the room.
Chase Doom around the room and pound him until you get all his health down for the final piece and you will advance to the end game cinematic.
After that you'll end up alone with your team back in an empty room and a save portal so you can save your status at the end of the game and either start over at a different difficulty, or do extracts so you can pick up whatever items you missed over the course of the game.