[Cerek the Barbaric]

I think I have the answer to your problem.

Pick up Minsc first and go search for Dynaheir.
When you rescue Dynaheir, send Imoen down into the hole to take her place.
Then wait patiently for the next gnoll attack and send Dynaheir into melee.
After she dies, retrieve Imoen from the hole and continue your adventure.

Granted, I'm not sure what Minsc's reaction will be when Dynaheir dies, but it may be worth a shot....and you're guaranteed NOT to lose Imoen.