I ran with a host and a summon through the township the other day and noticed the host hadn't killed a damn thing throughout the journey.
So we get to the Manus battle and the host stood as far away from the fight as he possibly could and would not heal us. After the other summoned died I said "screw this" and led Manus right to the host.
Watched in glee as the guy got torn to pieces by the Manus death rage frenzy. --- PSN Odinson31
Pull a Rosie and vow of silence the host or parry them while they're trying to fight the boss. Aggro enemies and run away and watch the swarm.
Some people seem to not know they cna heal phantoms... or perhaps they don't monitor phantom health. I always wait a bit to see if they'll use a heal.... But I try to get them before they burn humanity. But if they got heals... Use those dude.