Last boss fight on the Legacy of Bhaal difficulty

Is it actually possible without cheesing or cheating, or did I just completely gimp my setup? My companions are Corwyn, Baeloth, Dorn, Neera, and Rasaad, an my PC is an Elf Fighter / Mage multi-class. My gear is pretty decked out and I have all the spells from the game so, in theory, I should have all the requirements for beating it, but I'm finding this fight abnormally difficult. The only guide I was able to find was a 3-year-old YouTube video from the time when saving throws were still bugged, and he just used a Wand of Polymorphing on the boss and pretty much instantly whacked him, which makes all his summons disappear, leaving only Hephernaan, who all by himself is a complete pushover. I'm playing on the latest v2.6.6.0 patch where the saving throws are fixed, so I would basically have to hit him with Lower Resistances, Greater Malison, and Doom to even have a small chance of a polymorph landing successfully, but by the time I get all the spells off, my party is already completely dead. I tried this strategy for 3h one night and was only able to get a single successful polymorph off, with everone already dead at that point. I also tried a more "legit" way by entertaining the boss and his summons with a Wand of Monster Summoning while I picked out his goons one by one from max range, but the problem with that strategy is that he and some of his goons keep on teleporting right into my party and one-shotting people, which means I have to constantly keep moving with Haste on everyone 100% of the time, but that way I don't have enough time to attack and cast spells to kill anyone. I'm completely stumped here. Has anyone actually beat this boss on LOB because I would really like to hear their stories here. Also any tips on better strategies would be very appreciated.