Mission 2: Hunter!
This one has two parts. In the first part you control Chaos Space Marines and must hunt down Gorkutz while defending some temples against Ork attacks. In the second part you are switch back to Orks and destroy Imperial Guard troops and buildings (and possibly some Chaos Space Marines).
Chaos Space Marines
You start with Crull and Gorkutz facing off with some support troops. After a while, Gorkutz runs and you start getting objectives. Summon the Khorne Berzerkers with Crull like you're told and stard building squads and structures. I would make Marine Squads and 1-2 Horror Squads, and attach the Chaos Lord and Chaos Sorcerer to Marine Squads. Cover the temples with squads until you finish Fortified Positions on all your Strategic Points and Heavy Bolter Turrets to support them; then you should be able to gather your squads for an offense without leaving the temples undefended.
Now you can attack the Ork base a bit southeast from where you started with Crull. You can leave a squad behind in case the Orks send Stormboyz over, or just trust your turrets. Destroy the Ork compound – defended by Nobz Squads and other infantry – and stop to reinforce your squads.
Now continue to the main Ork base southwest from this one. You have to go up a ramp and down again to cross the wall between the bases. The Orks will mostly meet you with infantry and Wartrukks. Destroy the Bigga Generator you find upon arriving (a couple of Wartraks will spawn close by) and attack Gorkutz when he appears.
When Gorkutz goes down you get a cutscene and switch to Orks.
Orks
You take control of Gorkutz upon emerging from a sewer pipe. There's not much to this part – you move along the only way that is open, kill any Imperial Guard troops you come across (mostly Guardsmen Squads, a few Chimeras and a group of Basilisks) and destroy their Infantry Commands. When you advance you will find some friendly units and a Da Boyz Hut to commandeer.
The mission ends when the Imperial Guard are all dead. Some Chaos squads may reach the final outpost (located where the first Ork base was in the previous part) before the Guard are finished, and will be hostile, so you may have to kill some of them as well.