The quickest way to get this achievement is to begin either a Siege or Raid Skirmish Mode (Raid is better), when you head to the control/critical points, there will be drones defending the point, simply kill them 30 times (Will require multiple plays to get the achievement) and the achievement will pop since those are considered regular units.
In the Campaign mode regular units are the enemy soldiers that DO NOT have numbers attach to them like the standard units have.