You will find the ATAC in the Salamun Bridge level, Academy Rooftop checkpoint . You can't miss it. As soon as the battle starts, check which side the ATAC moves to. When he is in position over the fence shoot the blue lights with your rifle. The ATAC will get stunned, now quickly run up towards the stairs. there will be a Missile Launcher on the ground (on both sides). Equip it but don't fire. Now press to equip your Pistol, aim down the sight on one of the blue lights. As soon as they regain full strenght (full light), shoot it to keep the ATAC in stun-mode. Now take out your Launcher and pop one in him. Don't wait for the reload, just draw your Pistol again. Repeat said move, now reload, draw pistol, fire on the light. Draw Launcher, fire Launcher, draw Pistol etc etc etc. if you run out of missiles there are some on the other side of the platform, just keep the ATAC in stasis the whole time.