F.E.A.R. 3

This is what worked for me.

All Leapers (No weapon, Axe, Pipe, Wrench, Crowbar, Bombers, *Knife)

All Armacham Soldiers (Briggs SMG, G3A3 AR, EL-10 CAS, Sniper, Riot Shield) - Ghost Soldiers in Interval 8 count toward the above soldiers based on their weapons.

Phase Caster (Quick blue guy who summons lots of soldiers) * Must take his blue shields down first. * Note - Some people have reported that they had to possess the one at the end of Interval 2 and then one later in the game for it to register.

Phase Commander (Big blue guy) * Must take his blue shields down first.

All but one of these guys can be possessed in a split screen match of Soul King on Recruit with a dummy controller.

* I havent found any of the Knife Leapers on Soul King at the time Im writing this. You can find them in the Store level though in abundance.

first solution, so hopes this helps.
jagermeister tx has it right on the money on the types of enemies you have to possess.
i went to interval 5 where you have every kind of enemy that you need to possess **except the knife throwing Leaper** which you can get on interval 3. play on recruit, you will run into the leapers shortly after the beginning and in the middle waiting for the gate to open and every kind of Armacham soldier from the Briggs SMG to the sniper and riot shield guy.
you will also have a Phase caster in the middle and the Phase commander at the end of the level.
***for me at least i can confirm that i didnt have to go to interval 2 to possess that Phase caster. just did the one on interval 5 and went to interval 3 to possess the knife thrower and achievement unlocked***

hopes this helps you save some time

Very easy and quick - play Soul king mode, posses every enemy u see, phase caster and phase commander are here too, all kind of leapers and soldiers, after that (when u sure that u posses everything) go to campaign, doesn't matter which mission, possess the first enemy that u see and achievement pop up