[BUG] Can't finish the mission 5 "Guardian"

HI,
My friend and I were doing a private coop storry on this mission arrive to the final boss and doing fine when the gardian lock him self on invunerability state and stop moving here is a vidéo of it happening :

https://youtu.be/Z0Et7jR9TQ4

It’s probably cause we were to quick to destroy the Crystals but yet it’s steel annoying.