This garrison has nothing to do with Whitegate. Nor is the minimum level 75. The description above should be updated.
I had the pleasure of participating in this even last night. We had 13 people show up. So we made two full parties and rotated the spare person. A third party adds more mobs.
The usual technique of kiting one mob to cure friendly NPCs and recover MP works here. We had a BRD kite with very few issues for links. What XYZ advises about an outside party was not necessary for us.
Mannequin feet & legs fall like rain during this event. Success also gets 2*Exp scrolls, 2*5000 gil(if all friendly NPCs live), and 3*the weapons listed above.
Fun.