Credit to Cookie Of Death for the help.
To get this achievement you have to let the security guns on the levels kill the enemies, these are emplacements that are have green lasers and fire as anything crosses them.
This can be completed on any level, however there are more enemies in later levels. I completed this on 4-4. Save some friends and as the enemies start to spawn you can bring them to a turret and have them taken out by the turret. As soon as you kill 20 the achievement will pop.
It's good to have your companion control the shield as well and to play on casual to take less damage.