You will get this mission if you finish Final Journey with the Eldar. Your goal is once again to destroy all Monoliths. This time the Necrons attack right away, and the Disorder troops only come in at the end of the mission. This walkthrough assumes Insane difficulty.
You start with a base and some troops. First of all, avoid the Imperials. You gain nothing by fighting them, and they will fight the Necrons. Capture the Relic and the SP right below your base. There's another SP a bit southeast that you can get without fighting the Guard. Build two Soul Shrines for fast teching and several Webway Gates to raise your caps.
When the first Necron attack comes (from the south), make a defensive line just below your base. Don't go out, let them clear the Imperials out first. When the Monolith gets close enough, you can use the Soul Stone ability from your HQ to blast it. However, more Monoliths appear and you must take control of the Titan's weapons to defeat them.
You get the weapons one at a time by sending a Bonesinger to stand right next to the weapon for 60 seconds. Start with the unguarded ones, naturally. When you get a weapon, you'll notice that it can fire anywhere on the map. However, this time the Monoliths need about one more Titan shot compared to Titan of the Emperor, and they also "phase" (teleport back and regain health) when damaged, meaning you need several shots in quick succession to kill one. You can prevent them from coming close by forcing them to phase, of course.
When you use your first Titan weapon, Taldeer will tell you to attack the Monolith with your troops to kill it before it retreats. Don't. Keep your units alive and wait until you get more Titan guns.
If you don't get a cutscene with Taldeer and Sturnn agreeing on an alliance by the time you capture the unguarded weapons, you must take some units to clear one of the Imperial outposts for your third weapon. The alliance should come after this at the latest. After that, you can get any gun without fighting as the Guard turn friendly. With three guns you can kill a Monolith with a single volley.
Late in the mission, Chaos and Ork units start attacking. The strike that likely concerns you most is the first, performed by Chaos and coming from northeast. They have Defilers and will overrun the Imperial outpost unless you send some anti-vehicle units. Beating the Disorder attacks isn't necessary, though - you only need to delay long enough to blast the final couple of Monoliths.
When you get all the Monoliths, you win the mission. The Titan blows up and the Eldar run away through their Webway Gates. Campaign finished!