Because you need to play trough the game twice to obtain all achievements anyway, you will most likely unlock this achievement naturally.
Load up the level DEFENSE
in chapter four. After the first part where you need to defend the station with the squad called "Children of the Underground", you will make your way trough said station. Make sure to talk to the commander after the initial fight where he will give you a recording which you will need for another achievement unrelated to this one:
Now all the enemies that you will encounter in this level are called Lurkers. They are small ratlike monsters which will, according to Bourbon, only attack humans if they are traveling alone. Now in all areas they will spawn infinitely so if you wish you can just farm this out in the first big hall of the overrun station.
However you will kill about 15-20 during the whole level so if you take it slow you will probably unlock or be close to unlocking the achievement anyway. Like mentioned earlier, a second playtrough will definitely unlock this for you.
These little guys are fast so I recommend a good shotgun or some military grade bullets to take them out fast. Also do NOT jump after them into the holes as you will get stuck and die.