Meet Johnson early
From Halopedia, the Halo wiki
On 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.
