At the start of "The Great Journey" push Rtas 'Vadumee off the cliff and kill the Spectre driver behind you, but be careful not to destroy the Spectre. Then proceed through the Level as you normally would until you get to the part where you help Johnson clear the way, then get a Ghost and carefully push it off the cliff. Then use the Spectre to drive off the cliff. Get the Ghost and drive to the back of the Control Room. There should be a Platform with a beam next to it. Not to be confused with the larger beam on the side of the structure. Carefully drive the ghost up on the beam pushing against the wall and continue till you get to the front of the structure and wait for the Scarab to blast the door below you then drive down the slope and carefully fall to the control room door. squeeze yourself through the fiery hallway until you get to the control room hallway and let the cutscene go. Now go back to the hallway and grab the Ghost. Now kill Tartarus with some RAV Power!
Do exactly the same thing as the ghost except you can use the Spectre from the start.
Just fly to the control room door. Squeeze through the tunnel and let the cutscene go. Then go back to the hallway get your banshee and kill Tartarus with GSV Power!