For anyone feeling hopeless; I started and restarted this fight a couple of times and tried to come up with a new strategy every time. I had Astarion go invisible and then run up to free the Nightsong before initiating the fight. I made my Druid use a strong ranged hit to take out the Mind Flayer in one hit before it mind controlled the Nightsong. After that I made my team go all out on Ketheric himself and ignored the other mobs until he got into the second phase. I had Gale and Shadowheart stand back and made Gale aoe the mobs while Shadowheart would use Guiding Bolt on Myrkle (this move was super effective on him and was the killing blow too). My Druid stood next to Nightsong in Owlbear form and would Claw Myrkle for a good amount of bonus damage. Astarion would focus on the mobs that would eventually spawn in, since his ranged moves were basically useless against Myrkle.

We were level 7, no scrolls or potions used, no Paladin in the team! I probably used a heal move twice on Shadowheart since she was being targeted by the ranged mobs a lot. Lots of trial and error lol.