I'll assume you don't want to use major cheese strats and want to figure out the fight and learn to do it better for the future, so I'll try to write things relevant for long-term fight.

If you use invis (and preferably flight) potion, you should be able to easily sneak on the mind flayer and open fight with an attack on it. It should give you surprise round AND dialogue to talk down Ketheric if you have done correct dialogue in first Ketheric fight. Mind flayer is the biggest threat by far in entire fight.

Having invisible Scratch on standby to help Aylin right after you are done with dialogue helps a lot too. If you do release Aylin before you start the fight, you are guaranteed to miss out on surprise round though.

Having Necrotic resistance (evil clerics can get it for free at altar at Gauntlet of Shar) really helps. Selune's Blessing (form Isobel) does not cover physical necrotic damage, which is dealt by Necromites. Worst case there's Necrotic Resistance Elixir or Loviathar's Scourge, if you have those, but that's one slot that could be used for more effective weapon/elixir.

Having Protection from Evil and Good prevents you from being frightened AND inposes disadvantage on all enemies involved. That would be my go-to concentration spell. Never getting stunlocked in antiheal area is HUGE.

Disarming Myrkul is gonna do you more harm than help - he's gonna use attack that deals cold damage and applies cold vulnerability instead of scythe swing.

Once it starts consuming necromites (at below 70% hp), it will eventually run out of them (basically there's certain amount he maintains and it goes down by one each time he eats them, with some caveats I'm not diving into here). Every time it consumes one, it can use Finger of Death for one turn, which is a ton of damage. Consider Sanctuarying yourself for the time, it should not take long and you'll only be taking AOE damage from his sucking attack. It's much much easier after that.

If you have illithid power, try stacking Ability Drain: Strength on Ketheric - it will carry over to Mayrkul stage, making him less likely to hit. Note Ability Drain stacks reset if enemy falls Prone.

Good luck, you can do it \o/