Destroying Orin the easy way
Orin's gimmick is her 12 stacks of unstopppable (less in balanced and explorer mode), so the key to killing her is to get rid of them as quickly as possible. You also need Gale to go first which can be pretty much guaranteed by the alert feat or by taking an elixir of vigilance
I used Gale to unload a level 6 magic missile barrage at her (equipped with Markoheshkir bolts of doom enabled to add lightning damage), followed by taking a potion of speed and then hitting her with the Artistry of War which took a massive chunk of health off her.
After that, my Bardlock Tav just hit her with an Eldrich blast which finishes her off
This was in tactician, but I've used the same tactics in Honour mode with the same success