From StrategyWiki, the free strategy guide and walkthrough wiki

You will get this mission if you finish Final Journey with the Eldar.

This is on the same Titan map as Titan of the Emperor and you must accomplish the same thing – destroy the Necrons. There are some Imperial Guard nests around but avoid them for now. To keep it short, a Necron Monolith supported by infantry will attack. You defend with whatever infantry and vehicles you have until the Farseer tells you to blast it with the Soul Stone. The Monolith blows up from the first shot, and you think "What? That was it?" – but no, more Monoliths land and since the Soul Stone had only one shot, you are now screwed.

Now you must take control of the Titan's weapon systems as you can't use the Stone. This happens by moving a Bonesinger to one of the red spots, after which you must wait for 60 seconds to gain control of one weapon (you can only capture one at a time). Go for the two that don't have any Imperial Guard around them first – the guns can fire to any direction and distance this time – and hotkey the weapons when you get them. Start blasting at a Monolith… and more trouble. When you hurt it enough, it "phases", which means it teleports back a little and regains health. So Taldeer orders you to actually assault the Monolith with your ground troops so it can't recover between the Titan shots. You can do this, or just try to get more Titan guns (when you have 2–3 the Monoliths won't have time to heal).

While you're capturing the two unguarded Titan guns, get some troops and clear one of the guarded ones. After you get this third gun, you have a chat with Sturnn who relents to an alliance and the Imperial Guard leaves you alone, attacking the Necrons instead. Now you can get the final three guns without a fight, but you probably won't need them. As the fight goes on, some Disorder troops attack as well, but with three or more Titan guns you should kill the Monoliths so quickly the Chaos army won't even reach you before the mission ends.

When the Monoliths are dead, you win the mission. The Titan explodes because of the "strain" (explodes after a handful of shots from "minor hull weapons") and the Eldar evacuate through their gates.