Same here, just encountered it during my run with Boreal. Usually it can be removed with just shots from gun blade, either in the sphere or out of it. However in today's run, it does not work and had to let it run its course before I can continue. Worst is that before I can proceed to damage it, it proceed to summon 2 sentinels. After the sentinels killed, it goes back to the knockback sphere again.... What a pain to kill.