This mission is much the same as Final Journey (although with some changes) except you control the Disorder side. You can switch between factions at will. This is a bit annoying as you were probably hoping for some payback for dealing with endless waves of Chaos Marines and Orks, but now have to put up with endless waves of Imperials and Eldar instead. However, this time using both factions isn't quite as useless as in the Order mission.
You win the same way with both factions: you need to open the Psychic Gate and get someone through. You also need to accomplish this before the Space Marine convoy gets there (mandatory for both factions) and before the Eldar relocate their Webway Assembly there (probably not necessary for Chaos Space Marines, but it's easy to stop so just do it to be safe).
To thwart the Imperial and Eldar factions and stop the "fail timer" – so that you can take your time controlling the map – the easiest way seems to be two early rushes with both Chaos (for the Land Raider) and Orks (for the Webway Assembly). After this, you are safe to pick your preferred faction and go on about winning the mission. The rushes will be described first and the rest for each faction below that.
Rush to kill Land Raider and Webway Assembly
As you start with Chaos, start building 3 Heretics right away. Start a Chaos Armoury with one, a few Plasma Generators with the next and several Heavy Bolter Turrets to your north/northeast with the last one. The point is to get more Horror Squads as soon as possible and set up a no-babysit defense against light infantry raids. Attach your Chaos Lord to the Possessed Marines and take the closest two Strategic Points (the top one has Karskins around it so the Possessed go there). Do this very fast (you may want to pause the game and use the Esc key for skipping cutscenes) and switch to Orks, because their rush is more urgent.
With the Orks, select every unit you have and rush the Webway Gate right away. It will relocate faster than normally possible so you need to do this at once. Ignore everything else and go straight for it. When it's down, smash the rest. You can then leave your units there or move them back to your base. Don't loiter, switch back to Chaos.
Start building a Sacrificial Circle and put Listening Posts on your Strategic Points if you didn't before. Build turrets around the top one. Keep out of your northwest, you don't want to waste resources fighting with the Imperials there. Now start clearing the area where the big wall is broken (northeast from your base) with your Possessed Marines supported with Horrors, they should chew up the enemy turrets pretty fast. Through the wall, take out the Infantry Command, Listening Post and a squad of Ogryns on your right. Don't stray west or north or you risk further troops and artillery fire. You should be able to clear this area before the Land Raider gets there.
Now you should be in position to attack the convoy. The Land Raider will lead so engage the trailing Space Marines with your Possessed Marines and basic infantry, then attack the Land Raider from the front with Horror Squads (you can start when you have 2–3 and keep teleporting them in as you get them). If you can spare the Possessed Marines, tell them to attack the Land Raider as well.
When the Land Raider is dead, withdraw to your base. If you want to play Orks, build an adequate defense with Chaos before switching (so your Ork base won't be attacked by Imperials through the neglected and overrun Chaos base).
Chaos Space Marines
After killing the Land Raider it gets easy for Chaos. Start researching – you can get a couple Strategic Points at this point if you have units left from the rush – until you have fully upgraded Possessed Marines, Chaos Predators and Obliterators. Then just start advancing north and murder everything. Don't bother destroying the Ork base as you don't need to and it could break the mission leaving you unable to finish. If you want to take the east side of the map as well, just take some units past the Ork base from the north side.
You must open the Psychic Gate by capturing the Critical Location west from it, then have a big fight with remaining Imperial Guard and Eldar forces in front of the Gate. You can pick off their turrets and infantry from the sides with long-range units if you wish. When you get close enough, the Imperials will drop-pod some Space Marine Squads in there and the Eldar will summon their Avatar so this would be the logical place to summon your own Bloodthirster.
You win when you get someone past the Gate, so you don't actually need to kill all the enemies, but with maximum numbers of your highest quality infantry and vehicles you should have no problems with them and it's probably more satisfying to fight.
When you win you get the Titan of Khorne final mission.
For Orks, this mission is very frustrating. For some reason you get resources painfully slow – much slower than your Requisition and Power numbers indicate. It will be a while until you can do anything else than keep your few squads reinforced and build the occasional Listening Post. You can either start pushing north right away (struggling with your poor economy) or hang around your base (so you get no new Strategic Points and Requisition comes in even slower).
You must open the Psychic Gate by blowing up the Thermal Plasma Reactors on the east side and get someone through the gate. You don't need the west side of the map, but if you want it you should avoid the Chaos base and slip some units past (later, when you have the resources). For now, keep to the east because you have maybe two Nobz Squads and your commander.
So start going up and taking Strategic Points. Keep one group of Gretchins at your base and one with you to build Listening Posts and repair vehicles (if you have any). Keep them infiltrated. There will be places difficult to take with your meager forces so think ahead. At the first place where you find two Aspect Portals (near the second Eldar base relocation point) two squads of Eldar infantry will spawn periodically even when the portals are gone, so you'll need to leave some units to kill them every time they appear (say, two squads of fully reinforced and upgraded infantry).
Eventually you start building up resources, and you can bring your best units to the front. Don't make a Squiggoth if you don't want the west side of the map because he can't fit through the east side all the way. When you have destroyed the generators you will find the same final fight in front of the Psychic Gate. Again you could win cheaply by taking an unit through, but after all that annoyance you probably want to trample the Order good and proper.
When you win you get the Trophies final mission.