Enter a Scarab glitch

There is a glitch in which you can get on board the Scarab before the cutscene Backseat Driver.When you get to the platform, grenade jump to one of the legs. Then crouch jump onto the deck. You can now go inside the Scarab (there is no shield), and push the invincible Johnson to the edge of the Scarab, and get him to fall off the Scarabs deck, and down to the sand far below.

Alternate way
Warning:This require's timing

When Johnson jumps aboard it for the first time, before that, you must bunny hop to the edge of the platform. When you get there, jump. It is extremely important that you time this perfectly. As the Scarab moves back a bit, jump right then. You should end up going higher than usual. This can take a few attempts. If it works, you must jump towards either the left or right leg.If done correctly, you will land on the leg. Then crouch jump onto the deck.

Note: Pushing Johnson over the edge will stop the cutscene. Do not attempt this unless you want to revert to the previous checkpoint, or have already beat the game.