Instructions
Pick up the nearby rocket-propelled grenade on the rooftop immediately after the cut scene. Run down the stairs next to the RPG into the lower level and run into the tunnel beneath the platform.
Use caution as you take cover in the tunnel, crouching as you pass beneath windows through which the ATAC can shoot you. At both ends of this short tunnel are open areas surrounded by generators that emit blue light above the ground.
Go to one of the ends of the tunnel and wait for the ATAC to hover around the blue generators. Shoot any of the generators -- with your pistol, to conserve rocket ammo -- and an electric current will start flowing through all of them and stun the ATAC. Shoot the ATAC with your rocket launcher while it is stunned.
Retreat into the tunnel for cover once the ATAC regains control. You can pick up some rocket ammo in this hallway on your way to the opposite end, where you do exactly what you did before: Wait for the ATAC to hover around the lights, stun it and attack it. Three or four rockets -- depending on your difficulty setting -- will destroy the ATAC.