The final boss of Spirit Crucible Elpys is a phantasm that, whenever you get him down to 25% he goes invulnerable and multiplies. It seems there's a max of 6 at any given time. How do you kill him without him multiplying further?
I was able to kill him by following a certain strategy:
When your current phantasm gets down to about 40% health, stop attacking it and target a different phantasm (using RB+Y or RB+A). The NPCs will finish the phantasm you're targeting off.
You should continue to do this until you're down to the last phantasm. I think once you get under a certain number of phantasms they stop multiplying.
I ended up beating the fight by being slightly overleveled ,dogpiling ourselves into a corner and trying to take advantage of AoE arts. When we were down to about ~3 shadows left, trying to evenly whittle them each down to about ~40% health each with targetting, then starting a chain attack and trying to use AoE attacks to burst them all down.
Did this fight last night, lure them all into the poisonous water, they wont multiply when they are almost defeated. I found it out after 20 minutes of fighting and my tank fell down from the higher platform.
SPOILER* you gotta just keep fighting them... its part of the story. they eventually stop multiplying