This mission is quite easy. You must unite the Ork clans by capturing their bases and then wipe out a minor Chaos Space Marine base.
You start with an Ork Warboss, Gorkutz, and a Slugga Boy Squad. Reinforce your Slugga Boys, head for the first Ork base (Headcrushaz clan) and go straight for their big banner with all your Orks. Ignore their units and other structures (you get them for yourself when the banner's gone, so no sense killing them) and take down the banner. The next three (Rokkit Rangaz, Kwik Mekz and Burna Boyz clans) are just as simple. Get some troops, rush their banner while ignoring everything else and take it down. You can't research anything at first but you get new tech and options every time you capture a base.
For the fifth base you need a bit more. Send some Tankbustaz to blow up the enemy Bigga Generators and take a couple of squads of Shoota Boys or Slugga Boys to keep the enemy Stormboyz off them. When the generators are gone, rush the big Ork base, free the Mad Dok right away and send him to the Squiggoths. When he reaches them you get the whole base.
When you're done you only need to destroy the Chaos base. You'll have two Squiggoths and probably quite a bit of infantry at this point so you can overwhelm them right away, but if you wish you can build some Mega Armoured Nobz and Looted Tanks to make it even more unfair.
The mission ends when the enemy's Machine Pit and Desecrated Stronghold are gone.