The tracking for this seems glitched in regards to the enemy types which count. From detailed analysis it seems only the following enemy classes count:
Trooper (basic and Veteran only, NOT advanced or Pyro)
Killing an Imperium adversary along with other types above that normally count, will only count as one kill.
Behemoth bosses don't count.
Any robotic enemy doesn't count. That's the real bummer. So that precludes all enemies on the first level from counting towards the achievement as they're all basic robots or an imperium screen guy.
I will add to the above list of seven as my analysis continues but so far those are the ones I can confirm do count.
My tactic was to use the console save load trick when I found a group of five on the final mothership. Three were trooper / veteran troopers. Three kills for one group was the maximum I ever achieved. The rest of the group were robo soldiers or shield drones. You whittle the troopers down to the lowest health (I killed the robo ones first) then do a save. Kill them, reload.
So I was getting three kills every 30s or so. Try to do 30 sets a night, 100 kills a night while listening to music.