Meet Johnson early

In Halo 2 on The Great Journey, it is possible to sneak inside the Scarab. Simply put, you sword fly into it. Here's how. Near the end of the level (right before the Banshees), have a Carbine and Energy Sword. As the door opens to reveal the Scarab, use a Brute to sword fly into the Scarab. Seconds after entering the Scarab, Johnson will follow, equipped with a Plasma Pistol.

This can also be achieved by using one of the Ghosts and simply speeding out of the last doorway and climbing in before Johnson by using the Scarabs legs as steps. It may take one or two tries to achieve, but it should have saved a checkpoint and you can simply revert it if you fail. Also note that you can't leave the Scarab after you've done this.