How do you remove the refreshing stacks of unstoppable on Orin?

[Way]

title
the skull and cultists are invulnerable

[Prometheus]

i would reccomend just attacking with things that hit multiple times in a turn, like scorching ray or magic missiles

[Cinemax]

The cultists are not immune to AoEs. Don't kill them all if you want the achievement.
Crits seem to pass through unstoppable. IDK why this is, but most of my damage came from blowing past unstoppable with EB crits.
Low level multi hit spells can burn down unstoppable very quickly.
Spread so she can't topple multiple characters. Also spread away from the altar.
A fighter or zerker with low initiative roll can easily dumpster her after casters burn down unstoppable.

[Rommel]

Magic Missile for the win baby! That will shred that unstoppable crap realllll fast!