I normally capture the center CP. Defend it at almost all costs. I stay under the rebel's box and shoot them as they come down the hill. Make sure you turn around and clear out the rebels coming from the tunnels once in a while if your troops are getting overrun. If some of your AI players happen to catch up with you at the bottom of the hill, tell them to provide cover and charge up the hill. You can roll into the side walkway for a little cover while attacking. Take the CP by staying in the back near the healing robots, but watch out for rebels spawning next to you.
If you lose the three center CPs, there are only three main ways to your last CP. Get in a vehicle and stop them at the choke points.