Here you must destroy a) Isador and b) all Chaos units and buildings. There are three Chaos bases; directly behind the Relic "pyramid" and on both sides, to northwest and east from the Relic. All will launch mixed-force attacks so hopefully you perfected your defense in the previous mission.
This mission should be played a bit differently than most campaign maps. You don't want to turtle at all - most of the map is unclaimed when you begin. If you make, say, 2 Scout Marine Squads and 4 Servitors right away and start capturing SPs aggressively, you can have everything on your side of the river - around 10 Strategic Points and a Relic - before you even make enemy contact. This will give you plenty of Requisition and make it hard (impossible if you play it good) for the enemy to attack across the river at all. If you stay put and let the enemy get all the SPs, they will really make you feel it.
So - hit the ground running and start getting SPs. You need several Servitors to LP them while building a base at the same time. Try to fortify the outer LPs early to beat back early attacks (infantry) easily. You should be attacked by a Bloodthirster - their Relic unit - at an early point; deal with it by dancing a melee unit (e.g., Assault Marine Squad) around it while everyone and their dog shoots at it. Both commanders should be hacking at it as well.
When you have control of your side, don't stop. If you started early, you can hit the other side with a very modest force for a lot of damage. Nothing but a missile squad (for buildings), a Land Speeder (for infantry) and an infiltrated Scout Marine Squad (for extended range) can destroy the next row of turrets/LPs, and frankly you should have better stuff (i.e., lascannon tanks) coming already. Go over before they have anything good by the river and set up your defense on their side. Lots of turrets around your LPs like the last mission.