This mission is easy if you're quick at the start. Group all your units, and send them down and to the left, towards the symmetric's base. They'll probably meet about 2 4x4 pickups and 2 bikes, kill them. While this is happening, build two power stations to the top left of your base near the oil patch, a derrick, a few shot gun guys and an alchemy hall. As soon as you have done this, go and check on your units. Keep going down and to the left 'till you come to an opening to their base. Go left, kill the cannon tower, and destroy as many power stations and tankers as you can. Go up and kill their oil rig, then come back down, and make sure they don't rebuild anything. Go back to your base, build more shot gun guys, and send them down with your other units.
Research the clan hall, and put flamers on infinite. Build more power stations and derricks as needed. Send all infantry down to your other units. With your army, go right and kill the cannon tower, then their machine shop. Once you have taken and held this area, it's just a matter of time. Down lower and to the right is derricks and power stations defended by cannon towers. Build a beast enclosure and get beetles, and take out the missile tower at the top of the base, then you can sit your army here until you swarm through the valley and kill them all. Watch out though - the AI uses air strikes in this mission. There's probably a better way to kill their base at the start, but I'm doing this from memory. I remember it was pretty easy though.