Another great place to get this achievement is at the location that StrongProtector mentions in this post:-
It takes some careful aiming/timing but.... Try to javelin (or detonate if you're going for the detonate achieve as well) one of the respawning Necro's to the wall, then use kinesis to pull one of his claw/limbs and use that to spear the next one that respawns. I played this on normal and the Necros here go down in one hit, allowing you repeat the method until the achievement unlocks.
The position I used;
Just as you first enter the area you'll see a 3-seat bench directly in front of you (with another up-turned bench laying in front of it). Move just behind the bench and look across the concourse to your right, to the other side. You should be looking at a store called "Bold Stylings". One of the Necros will continually respawn to the right of where you're looking and run across towards the left. Try to pin them here.