The group I was with had great success when the 3 people outside the portals split up a bit. Have 1 at each portal killin supplicants and the third transferring back and forth to whoever needs help. Made keeping supplicants under control much easier so we could just spam atheon with bullets and supers without having to worry much about the adds Also standing on top of the pillars that open the portals is a good way of surviving the supplicants. Not 100% of the time, but pretty close