Hunter of The Plane of Justice Achievement Guide

 The enemies that you need to defeat for Hunter of The Plane of Justice are only found in two parts of the zone - the grallok tunnels and in the PoJ Trials. There's two named enemies per Plane of Justice trial which have a chance to spawn - you're usually guaranteed at least one named per trial from what I have seen, you'll have to run each trial a few times though to get them all.

All of the other nameds are in the grallok tunnels which are in the far southeastern portion of PoJ.