On the level 'Halo' when choosing it on campaign, choose rally- on co-op. When in johnson's warthog, wait till cortana says 'drive, cheif' and you see an explosion. Drive towards the explosion instead of turning right,but don't go on the panals- instead, turn left at this boulder, and you should see a mongoose. Whoevers on the turret can jump off and ride Johnson's mongoose (it's built up and tough.). Now, race away!