Atziri with multiple players

The wiki says there will be more empowered flameblasts at once with additional players, but how many exactly? How many players can there be before avoiding the flameblast is impossible?

Alternatively, how much fire resistance do I need to be able to take the hit? With 5k max life and 79% fire resistance it kills me instantly. A topaz flask would raise the resistance to 85%, but would that be enough?