While the solution to this is really simple, the method is quite difficult. I happened to get this achievement on my first playthrough against her (As I didn't know you could get rid of the cultists.)
Regardless, this is how the fight can be done, every turn Orin receives 7 stacks of unstoppable from the ritual chant. This can be quite annoying as any damage you try to do just get mitigated to 1 until you've gotten rid of her unstoppable stacks. Even then, she regains it all next turn.
So what can you do? Well, Orin's unstoppable mitigates all damage to 1. But, it does not prevent her from receiving status effects. Effects like bleed and poison are great at chipping away at unstoppable and eventually allow your party to pummel her regardless whether she receives those stacks. In addition, some attacks such as magical missile are EVEN better at clearing her unstoppable stacks as it's multiple firings in one.
Either way, Orin's fight with all her cultists chanting and granting her unstoppable can be annoying. But using status effects like bleed, poison and attacks like magic missile can very much so help. Keep your party healed and buffed and you should be able to bring her down and earn that achievement!
Good luck hunting gamers!