When yopu get to the first bridge on the assault on the control room, turnt to your left and Jump off (should only be done in 2 player cooperative). there will be a ledge. follow the ledge down until the end ofit. Kill one of the players. when the dead players screen goes "waiting to respawn" you then jump down onto the next ledge and try and slide off it. if you fail try again and again. so that the dead player respawns onto the next ledge than try and attemp to slide off the rock. than if succeeded go to the landing pelican. go upto it andf than press x. it will than take you to the scorpion tank. Use that of the warthog to go to the next door. when there get the banshee. and fly around and do what ever you want.