And then it hit me. What was killing me wasn't the exorbitant damage from the enemies; it was the fact that with Quickening Zephyr up on them, the casters in my party ran out of energy and could do nothing. My brilliant idea (which will probably seem obvious to some) was to equip Energizing Winds spirit on one of my heroes. I put it on Margrid, but any hero should do - I put only 3 points I put into Margrid's Beast Mastery, and Energizing Winds lasted a good 54 seconds, nearly enough to cover the recharge.
Now, all I needed was a likewise desperate Dervish eager to try new things (read: on the verge of paying a runner). I found one - had my pick of several, actually - and off we went.
We finished the mission with Expert's Reward, in just over 11 minutes. Had we not waited for our forms to recharge we might have gotten Master's, too.
You can call this a personal triumph, but in the three days I was stuck at Ruins of Morah, I could find no thread on beating it. I hope this helps some other desperate Dervish who comes across it