Though it has been under development for quite some time, the Helghast have gathered useful data during trial runs which has enabled them to make useful development modifications before the ATAC would go into final production. However, after the destruction of the SD platform on Vekta, the Helghast have been forced to push the ATAC into active service as part of their defense against the ISA invasion.
The ATAC first appears during the mission Blood Meridian, however it is only seen during a cutscene. The first official battle with an ATAC is fought by Sev after being stuck outside of Colonel Radec's office in Radec's Academy. If you fire rockets at the ATAC unstunned, it will dodge it. To defeat the ATAC you need to shoot the electric rods while the ATAC is in the vicinity of them. While the ATAC is temporarily malfunctioning, shoot at it or use the rocket launchers. Repeat until destroyed. If you need to find a decent cover, rush one level down and hide in the tunnels while Sev regenerates. The last ATAC is found near Visari's Palace, before the fight with Radec. The last ATAC can be easily dispatched due to proficient weapons (Grenade Launcher, LMG), distractions (ISA Soldiers, Team Alpha Members) and proper cover (Trenches, tunnels and tall obstructions).