To see Grunt Aerobics on Assault on the Control Room, grab the Banshee earlier than usual. From here, you can finish the level without having to open the big doors at the end or fight hordes of Covenant forces. Just park your Banshee backwards, so that the front is facing toward the ramp leading to the top. Once it is backed up as far against the door as it will go, get out. The Banshee should push you through the door, where all of the enemies should be frozen (since the door was never opened). The only things that move are little circles of Grunts that jump up and down in sync, one after the other.