This can be very easily done in the mission you first play as him, "Doctor in the House". Play through the level and you will come to a point where you see Human Torch surrounded by enemies. Before you cross the bridge to get to him, make sure you are controlling Thing. As long as you do not press Triangle to switch to Human Torch, enemies will infinitely spawn in this area. It should not take more than 5-10 min to reach 100 enemies defeated.