When you do the hail that starts the event, all of the NPCs run straight to the Gorgax and almost certainly are killed by the various other trash mobs.
I snared all of the mobs on the front side of Gorgax, then pulled them to the back side and rooted them.
Then when you hail, they charge Gorgax, and then path back to the start before the rooted mobs come back.
Double check your count on allies (5) before you finish the mission.