The first way to do this is to place a Warthog in a corner, with the front facing out. Use it as a ramp for other Warthogs. The other method is to place a Warthog in a corner, then throw a plasma grenade under it, followed by a fragment grenade. When it flies up, shoot it with a rocket to knock it up to the stands. Note: Occasionally the Warthog slides off. Another rocket can knock it up farther and, sometimes prevent it from falling off.