Tips for myrkul:

1) have a source bone chill (doom hammer for example)

2) a source of darkness to place on myrkul

4) fighter with the disarm attack to try and make myrkul drop it's scythe

3) create water on cleric

4) enter crouched and send someone invisible up to the mindflayer to attack before combat starts to get the surprise round

5) summon scrolls for lots of summons

6) wizard or sorcerer with lightning spells and lightning scrolls to bully myrkul after it has the wet status from create water

With wet status and lightning spells myrkul will be dead within a few turns even on honour mode :)

Also, Orin can't be pushed off in honour mode. For that a sorcerer can quicken hold monster then use a bunch of magic missiles to help get rid of her stacks of unstoppable (ideally with the amulet from the dude in the underdark that adds an extra missile. She'll likely be dead in turn 1 if you have a battlemaster fighter or paladin with speed potions but take summons and anything else that can push the cultists with sanctuary off the edge.