Fight Tartarus with a Banshee
From Halopedia, the Halo wiki
It is possible to Fight Tartarus with a Banshee, in the Halo 2 level, The Great Journey. This may take a few tries to get on approach and takes a great deal of patience, but is well worth it to certain players.
In the second to last part of the level where Sergeant Avery Johnson destroys the door with a Scarab, wait for him to shoot the door then fly into it. Though not required, it is advisable that you destroy the two wings of the Banshee before you enter the door. You can still make it with the wings, but you'll just damage them and it will take a longer time. After you get inside the door, start flying upward and then forward. You will fly over one wall and then get stuck in between two walls. When this happens, fly downward. Your Banshee will end up in front of the door.
At this point, get out of your Banshee, so the door will open. Get back in the Banshee and boost through the door but be wary of plasma grenades. A few Brutes will engage you, use the Fuel Rod Cannon to grind them into powder and fly in the room and through the doors the Brutes came in. When you do this, a cutscene will show Tartarus activating Delta Halo. After the scene finishes, you can either kill the Brutes and then run backwards past Johnson, or just get the Banshee right then when the scene ends. Your Banshee will be sitting in the hallway of which you can go into and fly through the pillars to fight Tartarus.
Tartarus will try to attack you, but a few Fuel Rod Cannon shots should finish him. This can be an extremely good method to use on Heroic and Legendary difficulty settings for new and inexperienced players. Also, do not simply run him over and push him off the edge of the structure , as he will vanish into thin air (well below where you could see him without the Banshee) and reappear in the grav-lift.
After killing Tartarus, the cutscene will play as usual, meaning this trick has no effect on the scene.