Though there already are 6 guides for this achievement, I still wanted to tell you the way I did it, which I found the easiest and earliest possible in the game. I did it on my 2nd playtrough on 1999 mode so really, you should have no trouble getting it this way.
Just after you get your magnetic shield, you enter the bar/church or whatever it is, the place with all the crows. The first room has a staircase in the middle, and on the left and right are 2 big rooms with on the left a bar and on the right tables with lots of fruit. It's impossible to miss.
After you clear this room you go up the stairs, and immediatly up ahead you see the "church"-room where the Crow is standing, speaking to the pilgrims (I think they are pilgrims but that doesn't matter), they're standing there with 5 of them right next to eachother and 1 extra on each side. If you just enter that room, don't jump down, just stand there from up top and cast a Devils Kiss (fire spell) trap right in between the 5. You should get 7 kills with it with no problem :) Then press start and reload last checkpoint, and do this 4 more times
I only had to do this 3 times because when I got those 7 kills, it counted as 2/5 in 1 trap and the 2nd trap counted as 4/5. Maybe cause the description says 3 enemies but you kill more than the double (7) so it counts as 2? It did for me, this is the quickest and easiest way to get it I think. Good luck!
Also, if you don't have the salts to cast Devils Kiss, as you see them standing from up top, take a left and look to the right, you'll see a trunk with a bottle of salts :)