I found it a very hard fight, but once I beat it it felt great. As others have said, there's no shame in turning down the difficulty if you've had enough. I just did that in another game that I wasn't enjoying the boss fights for.
I got to the Bhaal temple myself today, in my 3rd PT, and also got annoyed at what must be a bug (introduced since my lats patch 5 PT) , namely that "see invisibility" is not working on the Bhaalists that turn their visibility on and off each round.
I kept forgetting that my Tav had
See Invisibility for most of the game. I used Shadowheart's
Spirit Guardians to find them by sending her wandering near where I thought they might be. Occasionally chucked a bomb if I thought they were further away.
To be honest I probably got lucky with the rolls the time I won the fight. This is what I did:
Tav (halfling bard) approached from one set of steps with Greater Invisibility to get a good vantage point. Jaheira approached from the other side in invisible Panther form. Shadowheart and Minsc (gloomstalker ranger) waited on the top level. Lae'zel is the one strapped to the sacrifice table.
Tav opens the fight with level 5 Hold Person on Orin and as many of the cultists as she could. Orin immediately transforms to the Slayer but is still held. If her still being held is a bug, I'd say she shouldn't have been able to transform while paralyzed either.
Shadowheart & Minsc jump down with Feather Fall, with Minsc getting off the first round bonus damage before they can close the distance to enter melee. Jaheira also pounces.
After a few rounds when Orin broke Hold Person I had Tav start shooting her with arrows of paralysis poison. The other three were mostly clearing up the other cultists, with Jaheira tanking damage through Wild Shape. I think I also threw a few of them over the edge with those repelling arrows.
Like I said, I think I got lucky that Hold Person and the paralysis arrows worked so well, but damn it felt like an achievement when it was over!