That fight is also a lot easier if you focus on using a heavy hitter with a bludgeoning weapon doing damage. I made judicious use of Sanctuary, and made sure any vulnerable characters were out of range. You can use ranged weapons to lure Grym and change its threat assessment.

I cast haste on Karlach, giving her 4 hits every turn. She'd do 60 - 90 damage each turn, then have shadowheart cast sanctuary. All while making sure everyone is out of range so grym can't do much damage to anyone else out of frustration that he can't target Karlach. Others still sometimes get knocked down due to his Quake spell, but I was able to make it through that fight on Tactician without anyone falling.

Be careful not to trigger a melee reaction though, since that will cause Grym to recalculate his target and can mess up the plan.