First time I did this fight was on balanced. I found it annoying because one of the large undead would consistently push the closest party member off the edge of the rock, killing them in the chasm. I had to reload twice and then I just ran into the center taking some opportunity shots. My party was PC Gloomstalker, Wyll, Shadowheart and Karlach. I burned down Balthazar before he could do much, he got 1 turn before dying where he summoned his scripted adds, drank a speed pot and then ray of sicknesses some one twice. Shadowheart turned as many undead as she could reach on her first turn then it was a fairly straightforward fight.

Second time I did this was on Tactician. Party was PC Warlock/Sorceror, Astarion as a Gloomstalker, Shadowheart Light Cleric and Lae'zel. I jumped down and stealthed the party and opened combat with Astarion from ranged. Everyone else was undetected and I was able to get all of them into combat and attack from stealth. Balthazar died on the first turn before taking an action. Scripted adds didn't even show.