Here you start with Orks and are switched to Chaos Space Marines to finish the mission.
The Orks part is very straightforward. You have some troops – some (mostly melee) infantry, Killa Kans and Wartraks – no build capacity, and although you have some resources, your Squad Cap is already over the top so you cannot reinforce unless you get most of your infantry killed. Your goal is to kill Sturnn, who is found near the Field Command marked with red, meaning just a short way from where you start.
You will be met with a squad of Ogryns, lots of Guardsmen Squads, Heavy Bolter Turrets and some vehicles (mostly Hellhounds and a Leman Russ Tank). Just try to use your units reasonably; don't send Slugga Boy Squads to beat on Hellhounds. Keep your Wartraks back to hit the Sentinels and Turrets and try to save them for the Leman Russ, who should be beat with a couple of Killa Kans at the same time. Killa Kans are also good for the Turrets and, well, everything. Your heavy melee infantry and a passing Killa Kan should make short work of the Guardsmen Squads.
When you get close to the Field Command and find Sturnn, just focus on him to get it over with. Your Warboss and the closest Killa Kan should take him out effortlessly on Normal even if you're shot at while doing it. When Sturnn is down, the part is over and you switch to Chaos Space Marines.
Chaos Space Marines
With the Chaos, your goal is to "fill" the Blood Pit by capturing Guardsmen Squads with your Chaos Sorcerer and bringing them to the Pit to be sacrificed. You also have to defend against some Eldar infantry raids.
You start off with a proper base for a change, so start getting Strategic Points and teching up. The Eldar will usually attack with 1-2 squads of Guardians, Howling Banshees and Fire Dragons from your northeast and southeast so build some Turrets around your Listening Posts and fortify them as well. You can build Chaos Predators, Defilers and advanced infantry so the raids shouldn't be much of a problem. If you want more Strategic Points, there are scattered Ork infantry and vehicles around that you can easily dispose of. If you take a Strategic Point away from your base it's usually left alone so just claim them and move on. Upgraded Possessed Marines can own pretty much anything on this map.
When you're ready to go for the sacrifices, pick one of the four Imperial Guard bases in each corner of the map. The closest one (top right) seems to be easiest. Move close enough to a Guardsmen Squad with your Sorcerer and use the special ability pointed out to you. The dominated Guardsmen will now follow the Sorcerer, so run to your base and use Chaos infantry to draw Imperial fire, as they will now try to kill your dominated Guardsmen. Move the captured Guardsmen next to the Blood Pit and they will be sacrificed. Repeat this as many times as needed and try to keep your troops from killing any Guardsmen so you can keep coming back to the same base.
When you have all the sacrifices down, the Eldar will attack with their Avatar and masses of basic infantry. If you have stuff like Chaos Predators, Heavy Bolter Turrets, upgraded Marine Squads and Possessed Marines waiting you should positively slaughter their infantry waves, but the Avatar is invulnerable so you must hold on for a few minutes while your Sorcerer is transformed into the Bloodthirster. Now you can take the Avatar; you don't even need to actually fight him, as you are given a Bloodthirster special ability that kills him instantly.
When the Avatar is dead, you win the mission, so there's no need to search the whole map – you only need the Guardsmen Squads to sacrifice.