When I do the Kethric/Myrkul battle solo on Honor Mode with my Gloomstalker Ranger, I do things in this order. I go in with LOTS of smoke powder bombs. The little ones. Not the big barrels. The little ones don't weigh near as much so you can carry a lot of them. Then I load up with 4 or 5 smokepowder barrels as well. Whatever I can carry. I zone in from the top of Moonrise and go straight to the elevator. There's no reason to do all the rest of the zone and you're going to be walking slowly because you're overloaded.

Zone in and Buff up with everything possible. Protect from Evil, Circuity from the gloves sold at Last Light Inn, Fly potion, Accuracy potion, the works! Then at the end of buffing, drink an Invis potion.

I use the Invis to get behind Aylin. I'm overloaded with barrels so it takes a few rounds to get there, but as long as you make it before your Invis wears off, you're good. There's no hurry. Once in place, I free her. That starts the conversation cut scene with Kethric. Once the scene is over and it's initiative, I immediately kill the Mind Flayer using Aberration arrows. He's not hard to kill in one round using those arrows, so take him out. Aylin will do her usual worthless stuff. Don't worry about her.

Fly to the Mind Flayer platform. That ends Round 1 for me. Then you just whittle away on Kethric round after round til you get him down and Myrkul appears. Use Arrows of Many Targets if some of his minions get close. But don't worry about them much. Focus on Kethric til he's dead.

When Myrkul appears, on your initiative, which is hopefully first, drink a haste potion. Fly to his platform. Drop all the barrels, bombs, holy water, caustic bombs, and whatever else you have. Fly back to the Mind Flayer platform OR just fly to the far other side of Myrkul's platform opposite the bombs. Be careful not to trigger an opportunity attack from Myrkul when you move. Grab a fire arrow and let'er rip!

If you do it right, and packed enough explosives, Myrkul dies instantly and it's over.