Just be clear, you can get the 30 kills in any combination of the above enemies.
If you enjoy playing sniper, then you will enjoy this achievement, if you don’t....well this achievement may suck.
I did this solo starting from wave 1 casual on Fallout. Add as many headshot and damage buffs as you can to make life easier.
Drag the fabricator back to the starting spawn and just try and get as many headshots as possible on the mentioned enemies. DR-1s sometimes need 2 headshots to die, but most will die in 1 with called shot etc.. I prefer solo for this achievement because all the enemies are facing you hence their big fat heads are ripe for bullets.
As a side note, it doesn’t matter if you fail at any point, as long as you continue and finish it all in 1 session. If you are a decent shot, you should net this achievement before the boss wave.