Other places to do it, if you have Obsidian powers:
Near the end of Level 7, there will be a few areas with a large number of enemies. If you have chosen the X-Men side, you will accompany a mech that Forge has hacked. There will be more than 150 enemies that will spawn during this part of the level. Once you have enough energy to become the Obsidian Titan, transform and attack. Your ground-pounding powers will instantly kill multiple enemies and rack up quick combos. In one turn as the Obsidian Titan, I managed two Insane combos, plus one more after another transformation.
Shortly after that, there is an area that has infinitely spawning enemies. You are supposed to head up some stairs while your teammates (regardless of which side you chose) hold them off. An Obsidian Titan transformation here garnered me another couple of Insane combos. (This is also a really good place to grind the Goon Squad achievement for killing 500 MRD troops. I did my final 200 in ten or fifteen minutes).
Immediately after that, once you head up the stairs, there will be a narrow path along which a number of the mutated MRD troops (the guys with the things sticking out of their bodies) will spawn. You can go either with the Obsidian Titan (just start smashing) or the Boulder Dash (just start rolling). Either one will net you an Insane combo.
And of course, if you wait until the end of the game there's that large arena full of enemies that you fight during the credits. One transformation into the Obsidian Titan should do the trick.
Good luck!