Instructions
Select the "Halo 2" level the "Great Journey." Park your Spectre by the entrance, making sure the vehicle is not destroyed. Progress through the level as normal until you encounter Johnson piloting the Scarab.
Land in the Scarab. Push Johnson from where he sits and continue to push him until he tumbles from the vehicle.
Return to your Spectre at the entrance of the building and pilot it to where Johnson fell to the sand. Align the Spectre so that the turret seat is directly next to Johnson so that your unsuspecting victim enters the vehicle's turret seat.
Use both the Spectre's air brake and speed boost to hop over the rocks and into the Brutes. Enjoy the backseat driver cut scene.
Drive yourself and Johnson through the control room, under the water, through the control room island, past the control room exterior, straight past another set of Brutes and into the control room.
Land on the platform area where you fight Tartarus. Position the Spectre so that Johnson is directly in front of a hole that drops down to the bottom floor. Get out of the Spectre.
Climb into the Spectre's turret where Johnson is. Johnson will get out for you and, in doing so, will fall through the whole. You have killed Sergeant Johnson.