How do I kill the Phantasm boss that keeps multiplying?

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.