Zardnaar
Legend
You can bring Glut and Bulette for that fight if you Long Rest at the top of the tower with both in your party. Provided you collected all the sussur flowers and place them on the floor next to the extra enemies before starting, the only thing you have to fight is the boss. Also the boss's Legendary Action will chain to and damage the extra enemies, so have a ranged character stand next to one and shoot the boss at the top of the round if you want to reduce your time spent a bit.
If you want, there is also an interaction between the Bulette's knockdown effect and a Cleric's Command:Halt spell. If the boss failed the save on Command, it can't get up again after Bulette knocks it prone. I don't remember what my WIS was at, but my Command:Halt had a 75% success rate on that boss. It is also very good for the Owlbear(s).
You can also start with Command: Drop and then steal the boss's weapon off the floor to remove it from consideration.
Various ways to do it. Normally just go with lightning resistance and beat them down.
It's one of those fights tgat can go pear shaped. Life ckeric with healing gear also makes it easy.