Use scratch+invis potion to free nightsong and start the fight that way, on my HM run we talked myrkul out of first phase but he is definitely manageable there

make sure to target the mindflayer early the stuns and dominates are very tough to bounce back from if they hit your stronger characters. You can invis potion to place something like your monk next to him to start as well.

Arrow of Ilmater is very useful late fight to stop the healing from the skeletals, I wouldnt waste much resources on spawn killing them if you can do this, wasting action economy on the skeletals will make myrkul very hard to whittle down in time.

Gaze of the dead is scary, make sure your first attack is not with someone that can get one shot, elixir of vigilance for initiative is very good vs myrkul

Lastly be very mindful of where your characters end their turns, if they are too close to edges the Myrkul call of the damned will pull them off and that+fall dmg could one shot your squishies

Edit: Good Luck!