Here's the best advice I can give.
1) Have a character sneak up to rescue the Nightsong before the fight begins.
2) Take down all the additional enemies before tackling Ketheric. The Mind flayer should be priority number one.
3) Ideally you'll have a Paladin, Fighter, Monk or Barbarian. Cast Haste on them one way or another. They will probably be able to down Myrkul in a round or two. Have everyone else deal with any additional enemies being summoned. If one of your characters is concentrating on Haste then you need to keep them protected, keep them away from Myrkul and ideally cast Sanctuary to keep them healthy.
If you don't have any of these consider dropping the difficulty down to explorer and use whatever sources of damage you do have to take him down as fast as possible.
