There are two main portions of the game that have many enemies to throw off the edge. The first is in the first mission, the Helicarrier. There are two parts where your team travels down on an elevator. To grab just press X and move the thumbstick in any direction that is not on the elevator.
The second is in the mission where you must fight Galactus. You'll know when you're on the right part when you see Galactus in the background and the camera is on level with the pathway you are on. The pathway is open on all sides, and just throw them off the edge.