A quick way to get this is in the last mission of Chapter 2, "The Capture of Togoreth Stronghold".
Just use a strong character you like, and rush trough the first zone (I kept pressing
to go faster), killing only the leader.
Then, rush trough the second zone (No killing needed here), and quickly destroy the summoning wall in the next zone to go forward (Don't take the stairs).
Next zone, only kill the leader like in the first one.
After that, you'll be in a room with a save spot, but more importantly three corpses with Knowing Tags. Then, quit and repeat until you get the achievement.
I used Sice at level 34 in cadet mode and I was able to kill every ennemy with one shot, each run, counting loading and preparations time, took 2-3 minutes.
If you haven't done the mission yet, it should be even faster as you can just save in the room, take the tags, quit, reload.
Hope it helps ^^