A Clash of Leviathans
Gorgutz makes one final push with his forces to the Spear, through both Eldar and Space Marine forces
Mission Prep
You are forced to have Gorgutz 'Ead 'Unter and Beauty da Morkanaut as two of your Elites. The ideal army you will use below consists of a lot of Vehicles so choose a third Elite and Doctrines accordingly, but given the kind of army you'll be able to build at the start nothing is truly critical.
General Notes, Hard Difficulty
At the very start you land with Gorgutz and Beauty just south of your base (which you cannot yet control, including the existing troops in there) with a number of infantry squads, and your base is under attack. Just aggressively rush in and attack everything, you won't need much strategy here and just defeat the enemy forces.
Once this is done the main buildup phase begins. There is a large door just north of your base which Beauty's Rokkit Fist can destroy, DO NOT DO THIS YET!!! While the door remains intact and you don't fly any Deffkoptas out, nothing will attack you. It is also an excellent starting base; two 4-node PoCs with an even mix of Requisition and Power nodes, a MASSIVE amount of Scrap of varied sizes allowing for every size of Vehicle to be salvaged from it and plenty of smaller ones for gaining upgrades, and nearly every key building; the only things you are missing to have a fully maxed structure set are a single additional Waagh tower and a Pile o Gunz for tech upgrades (you don't actually start with any Gretchin, so build a couple of those to begin with).
Don't initially build or salvage any more units other than the initial couple of Gretchin, and just max out all of your Tech in the Pile o Gunz along with having your existing infantry and Killa Kans clean up the base a bit by grabbing the smaller wrecks (even Space Marine Drop Pod remains count); avoid having your units break up any of the Space Marine tank wrecks for simple upgrades as those can be salvaged into Deff Dreads or Big Trakks.
Once all the tech has been upgraded, just wait a little longer for a decent amount of resources to build up (particularly Power), then build up a total of at least 4 Gretchin squads and have them start salvaging the large wrecks (either from the initial combat or your Waagh towers) to build up a massive force of Deff Dreads and Big Trakks. Then once this is done and the vehicles have had some initial repairs by the Gretchin, have Beauty use its Rokkit Fist on the door and - as Gorgutz so aptly put it in the intro - RAMPAGE OUT!
To the east is a Space Marine base, to the west is an Eldar base. Destroying either one will trigger the final combat stage where both of them are involved in a large final battle and the one whose base you destroy will be weakened. The Space Marines are definitely far more dangerous, so charge east; leave your infantry and likely Gorgutz back at the base entrance for defense. If you built up Deff Dreads and Big Trakks as mentioned above, even on Hard difficulty this really isn't that hard of a battle; such a force will mow through everything in sight, and the Gretchin can salvage Tank wrecks for more Deff Dreads or Big Trakks as needed.
Once the main base is destroyed (or the Eldar one if you still decided to take that on), the other enemy base vanishes and the final battle zone is revealed. A 15 minute timer starts which shows how long it will take before Autarch Kyre claims the Spear. Charge ahead to the final area, destroying the occasional forces you encounter on the way.
Once you get close to the final area, the Space Marines will warp in an Imperial Knight Paladin, and the Eldar will warp in a Wraithknight. You will need to destroy both of these Elites to win the mission; don't take too long, both sides will get endless, regular reinforcements by dropship or warp which will eventually wear you down, and don't forget you're still on a time limit.