The Great Journey
If the cutscene at the Scarab doesn't trigger, check if a Brute fell off the ledge and survived. All the Brutes must be dead.
Kill the Elite who brings forth the Spectre. Drive ahead and park it behind the rock up ahead, hijack a Ghost. Or leave the Spectre where the Elite parked it, which will guarantee that it doesn't get destroyed when you leave it behind.
At the door guarded by 2 Wraiths and several Brutes, you can get rid of them by making the game deload them. Basically drive up to the door, get to cover, face away from the enemies and wait.
Easiest way to ensure the deload is to backtrack and then come back. You can delay the CP to get it closer; on your way back to the Wraiths by looking downward and shooting at the ground. Another way to do the deload is to take cover near the Scarab platform and wait 10 seconds while looking away from the Wraiths. You can fire the Ghost's turrets to delay a CP.
Drive into the building and past the enemies. When you've reached the garden room, drive the Ghost across the gap. Hold A to lift the front of the Ghost up. You don't want to hit the Ghost's wings on anything and Brutes can be very annoying when they get in your way. It helps a lot to get out and cloak before attempting this jump. If you carried over a sword from Uprising, you can sword fly off of a Brute to the other side.
For the bridge, killing Jackals risks spawning more Drones. Park the Ghost on the left side of the bridge, get out and cloak. Drive across carefully, shooting the Jackal sniper with the Ghost. You can snipe the Jackal sniper before attempting this. If you carried over sword, you can cloak and sword fly to the Jackal sniper.
Driving the Ghost: Release the Hunters, kill 3 Brutes to open the door and drive past the reinforcements. Sword strat: sword fly to the Brute closest to the Hunters, stick the left Brute, cloak and backsmack a third Brute.
Outside Scarab: Shoot all the Brutes down with the Ghost. Sword strat: sword fly to the closest Brute, stick a second Brute, and backsmack the Brute shot Brute to make the last one berserk. Do not move or shoot so the ending cutscene happens faster.
Deloading Wraiths: Immediately after the cutscene ends, go back to the previous room. If done correctly, the 3 Wraiths should have their drivers removed from the game, allowing the Scarab to proceed. Make sure you have at least 2 plasmas.
Spectre Drop: Drive the Spectre to the spot that allows you to enter the control room early. Wait for Scarab to blow up the door, then park the Spectre near the edge, facing the structure, and get out. Time it so you trigger the cutscene as the Spectre is about to fall off. If done correctly, it should be waiting for you in the backroom.
Tartarus: The start isn't very hard, but it's possible to be killed by the 4 Brutes flanking Tartarus. If you have a spare plasma, you can toss one at them, cloak, and jump back. It will usually help you get out of that situation.
Get Johnson on the Spectre. Make sure to kill non-gold Elites so they don't get in. Park the Spectre at the edge of the platform where Johnson runs to. Johnson should respawn at the same time as Miranda, look at where Keyes will spawn. When you have 2 or 3 Johnsons in your Spectre, just drive down to Tartarus, stick it with a plasma, and enjoy the carnage. Dual wield plasma rifles and shoot Tartarus.
Sometimes Johnson will not clone and reverting will not change this. A good backup strat is to get Miranda Keyes on the turret, drive to Tartarus, destroy the Spectre, and allow her to distract him. It will take longer to kill him but it will still be an easy fight, if nothing goes wrong.