You have to shoot the leg joints until they give way, at which point the scarab will fall close to the ground. At this point, jump onto the Scarab and proceed down the scarab levels until you reach the bottom, where there should be a shield blocking something that will turn your reticule red. Shoot or melee the shield until it breaks, and then melee the thing behind it. After it breaks, get off the scarab as quickly as possible and get away from it before it explodes. In the Halo 2 version, jump on the scarab, and kill everyone on it. Go down a ramp, kill the people in the room, and walk to the end of the room (the side opposite the ramp). The game will automatically go in to a cutscene.