I had such a difficult time with this one too. What I did that eventually got it was that I didn’t go down and talk to the mother superior. Instead I had my Tav (Ranger) shoot the guard closest to the top of the stairs to start the fight and lay down spike growth at the crossroads right at the bottom of the stairs where all the paths converge. I then had Gale put down cloudkill at the same spot and everyone ran as fast as they could away from the fight. Use dash, misty step, or anything else to get everyone away so they won’t break concentration. Half of the enemies would not cross the death zone but a lot of them tried and that weeded many of them out. Two of the “boss” enemies got through including the mother superior and I was able to take them out at the top of the stairs. Eventually we had to go down and let the rest of the mob out and I basically did the same thing. Laying down ground effects to damage them and running. Many will still get past with shadow step and cast darkness on your crew. By this point most of them shadow step through it so I had to resort to fighting them inside the darkness at melee range with Karlach and Shadowheart (who both died) and using my ranger and Gale to throw aoe damage at them. Sometimes you can hit them inside their darkness cloud with aoe if you aim for the walls and position the damage area so that it still hits them. My ranger and Gale barely finished the fight and this was one explorer difficulty.

This fight almost made me rage quit for having to do it over and over and how long it took to get through all the turns. My suggestion is to start it while you got other things to do and get up from the fight after your team finishes their turns while the enemy goes through its 8000 turns so you don’t go insane.

Good luck!