The Agitator is a medium-range plasma artillery battery with a large AoE. It can switch between low and high trajectory mode, allowing it to directly target units or bombard those behind obstacles.
Build it once you have your front line established to push back your enemy.
Being quite expensive in T1, it's less useful as a defensive weapon than it is an offensive sniping tool. Position it to destroy frontline Metal Extractors, Radar Towers, Jammers, and Construction Turrets to deny support to your enemy.
Use scouts, frontline radar towers and Beholders to spot targets.
T1 artillery batteries are powerful, but will tie your resources down considerably. Consider Reclaiming them once no more easy targets are in reach.