Another campaign achievement.
There are 27 AU's in total, and you have to activate all of them to complete the campaign mode anyway. They're also bunched into groups of three and when you complete a group, you'll then be able to shoot off and detonate the associated beacon if you want.
This really couldn't be that much easier. Locate the AU on the map, run up to it killing any nearby resistance (there's normally a couple to half a dozen Cel operatives on the platform with the AU) and stand on one of the four switch on the platform until the AU is fully configured (there's a meter in the top right corner of your HUD.) Rinse and repeat for each of the AU's.
Like the beacons, you don't have to worry about causing any damage to the AU, so if you want to go in with explosive weaponry, go for it...
It might be easier to clear out the Cel strongholds in the region first as there's at least one that can be quite risky (lots of grenade launchers and homing missiles), but then if you're wanting an easier life of it, I'd recommend clearing all the Cel strongholds before doing any of the rest of the work anyway!