I'm gonna guess monk, since that's the class that lets you fight effectively without gear at low level, and it tends to be a natural fit for Astarion even if you don't respec him.
(That said, you can switch off between "camp clothes" and "adventuring clothes" -- the Astarion in the image could actually be kitted out in plate for all we know.)
Another option is to blast the chanting cultists. Once they're down, Orin's Unstoppable quits being refreshed every round. It's challenging since they're all under Sanctuary, which means you have to use AoEs or other indirect attacks (like @AdmundfortGeographer's Arrows of Many Targets) to kill them. The easiest way to get rid of them is to use effects like Thunderwave or Gust of Wind to throw them off the platform into the pit -- don't worry about the loot, they have next to nothing.
You can also knock Orin off the platform, but unlike the cultists, she has loot worth collecting, so I prefer to avoid that route.
I think there's an option to duel her one on one where she doesn't get Unstoppable at all, but I haven't tried that route yet.
