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.