I'm getting worked going through Zhentil Keep in Pools of Darkness right now. The problem is encounters with up to a dozen Drow priests and sorcerers who can all cast hold person. Their magic resistance and high HP means you can't kill them reliably before they all start casting hold person then half the party is held.

Anyone have tips for this? Is there a Ring of Free Action in this game that I missed?

You need high level mages who can cast Fireballs to disrupt their spell casting. The higher the level of the caster the higher the change of bypassing the Drows' magic resistance.

Another option (not very reliable) is to cast Silence on one of your own characters and move him next to the spell casters.

Spells like Protection from Evil, Bless and Prayer improve Saving Throws. So do Rings and Cloaks of Protection. Item benefits are not cumulative, though.

And if all else fails, and if it's possible (I can't remember) do your best "I'll be back" and explore somewhere else first.

It's a level 2 spell. You can cast Globe of Minor Invulnerability, and that character will be completely immune to it.