Head through the Scablands and clear out some of the Cabal before moving to the entrance of the Firebase. This area belongs to the Blind Legion and is full of Cabal. The area is large with little buildings to duck into and enemies popping out of them. Focus on the Centurions when they appear; they definitely cause the biggest ruckus. Wind through this area to reach the Rubicon Wastes. As you cross the observation tower you’ll find more Cabal but the biggest threat is the Colossus firing it’s Slug Thrower from the top of the opening at the back end.
Defeat the enemies and move up into a garage some way down. Pick up an Interceptor; it’s a very powerful vehicle and you’ll want to take it with you to the next area to make it easier. Burst your way through the Centurions and another Colossus to reach the Iron Line. You are invited in but it’s definitely an ambush.
With the Interceptor, just start letting everything have it. Don’t think you are invincible though; know when to bail. The second wave features a Centurion and the third wave comes from the right with a Colossus. Use your Ghost to open the gate once you’ve defeated them then start to storm the keep. You can’t take the Interceptor with you anymore so head through the gate on foot. Here you will be met with a Major Phalanx and two normal Phalanx. Be sure to hit their arms and take them out quickly. Make your way up into Legion’s Keep when the path is secure.
Here you’ll have to deal with Cabal from numerous areas so take it slow and wipe out one side at a time. The Colossus in the back can be annoying; hit it with a Heavy Weapon and Super Ability to get a breather. Ahead are more Cabal anchored by a Centurion so make sure to get a free shot in on the big guy! Deploy your Ghost to wake the eye then take cover. You can duck under the spire to avoid a lot of damage but Primus Shaaull , an Ultra Centurion, is no joke. Defeat the adds so you can focus on avoiding the Primus. More Cabal join at 70% and 35% health so keep that in mind. Save the Super Abilities and Heavy Weapons for the boss. Return to your Vanguard Mentor when you have won.