This is the boss for the "Salamun Bridge" level.
The fastest way to defeat him is the following:
-pick up the rocket launcher by the center pillar.
-when the ATAC is hovering above/near a blue capsule, shoot the capsule with your sidearm until it explodes. This will stun the ATAC.
-immediately fire your first rocket shot, and don't miss. You'll have time to fire a second shot and hit him if you do this fast enough.
-after the second hit, the ATAC will begin moving and attacking very quickly again.
-repeat the same strategy with the other capsules. The attack takes 3 hits I believe, but may be more on higher difficulties which I have yet to play at this point.
After a few tries you should definitely be able to do this no problem.