Scarab boarding glitch
From Halopedia, the Halo wiki
Start the level from the beginning. When you reach the first Ogab'd-pattern anti-aircraft Wraith, kill all enemies except the Anti-Air Wraith's driver. On Easy and Normal, the AA Wraith's pilot will be busy trying to kill the NPC Hornet, so meleeing the side "wings" off of the Wraith and hijacking it should be an easy task. Drive the Wraith into the next lakebed and kill all enemies.
When you reach the Judgment chapter, save and quit. Then resume the mission. After this checkpoint, do not save and quit.
Position the Anti-Air Wraith where the Troop Transport Warthog is. When the Scarab's leg comes down on your position, press LT to boost; the Wraith will be sent flying into the air. If you position the Anti-air Wraith precisely, you have no need to boost as the Anti-air Wraith will bounce off the crane. If you dismount the Wraith at the right time, you should land on the Scarab's deck. This may take a couple of tries, as the Wraith's position is a significant factor; if you do not end up on top of the Scarab after many tries, then press the Xbox Guide button, sign out, and resume the mission. Otherwise, revert to last checkpoint.
If the Wraith does not move from its position when the glitch is performed, the glitch may overload the map. This will lag the game. Eventually, the overloading will remove all NPCs save for the Hornets and Pelican (and that does include the Scarab); sometimes, it will even prevent the Arbiter from spawning.
Start the level normally. When you get the Mongoose use it to take out the Wraiths like normal. When the Anti-Air Wraiths are down, take the Mongoose to the crane next to where the Marines are shooting the missile pods. It is advised to detach the Missile Pods to prevent the Marines from accidentally killing you. There are two lifts. The one on the left is the one you need to use. Park the Mongoose on the lift, then get off and hit the button. This should activate the lift taking you to the top of the crane. Up there is a dead Marine with a sniper rifle. You can use that to help take out the Ghosts. Otherwise wait untill the Marines finish them off.
While you are up there, the Scarab won't notice you easily. It's best not to have the Marine with you because it can get the Scarab's attention.
The crane stretches long enough to give you momentum to reach the Scarab. From the top of there it's mostly about timing. Most of the time it will come around and stop near the crane to shoot the Marines. That is the best time. Sometimes it will go a different path but you should be able to make it most of the time.