You will get this mission if you win Final Journey with the Imperial Guard.
You will start with a base built on top of the Titan, surrounded with Strategic Points and Slag Deposits. Your goal is to build six Thermal Plasma Generators to power the Titan's weapons, which you will use to defend against the Necrons, who will attack when you have finished four of the generators. Meanwhile you will be harassed from Chaos Space Marine bases to your northwest and southeast and Ork bases to your northeast and southwest. To your south there is an Imperial Guard depot under siege by Orks where you can get a free Baneblade by repairing a damaged Mars Pattern Command.
Start by scattering your infantry and capturing the Relic and all Strategic Points as fast as you can. You should build Listening Posts quickly so you can tunnel-transport infantry around to answer the attacks; the nastier ones come from the Chaos Space Marines (NW and SE). You can safely build three Thermal Plasma Generators so you should have plenty of Power. Build more Heavy Bolter Turrets when you can. Start your research; a couple of Karskin squads will really help your defense. As for vehicles, skip the weaker ones and go straight for Leman Russ tanks.
While you're scrambling to build up and defend your Strategic Points and generators, you also want to get the southern depot before the Orks overwhelm it. Also, the earlier you get the Baneblade the easier it will be for you. If you have trouble detaching your light infantry from defending your base, just break through with your Command Squad, Chaplain and Space Marine Squad, and tunnel-transport a Techpriest Engineer over to repair the Mars Pattern Command and blast the Orks with the Baneblade. After this, you can either pull back to your main base to defend and build your generators (after which the Necrons arrive and annihilate everything anyway) or, if you want to prepare in peace, take the Baneblade and some support troops and destroy the enemy camps.
When you have finished the Thermal Plasma Generators (just build the final three at the same time) the Titan's weapons are activated and Necrons attack from all directions. They will come with superior infantry and Monolith moving bases and totally kick your backside if you don't use the Titan. Hotkey the six Titan guns to 1 – 6 for ease of use and blast the Necrons when they come in range. You can slow them down a bit with Karskins (who have superior speed/range and grenades to scatter squads), Leman Russ tanks and of course the Baneblade. Building lots of extra Heavy Bolter Turrets in advance will also give the Necrons more targets.
You win when you have destroyed all Monoliths.