Here's the strat:
There is the entrance area that has about 4-5 Mamools on the bottom level, while you're on the top, avoid them. Go straight to the eastern part of the map. There are 6 'corridors' for lack of a better word. If memory serves me correctly, there are locked doors at the central north, western south, and eastern south passages. Your objective is to break down these doors and open the pot hatch behind each one until you find the NPC. He won't be detectable until AFTER you open the hatch, so don't try to tab like crazy trying to target him.
Now, if you to pull a Mamool WITH A WEAPON to one of these locked doors, with your tank's back to the door, so that the Mamool's attack goes past the tank on through the door. The WS you're looking for it to do is either Axe Throw or Staff Throw (or something to that effect), depending on which weapon they're holding. It is possible for Mamool's to lose their weapon before doing the WS, after which they are useless and should be killed ASAP. Also, note that after using the Throw move, they will lose their weapon, so kill them after that. A couple mamools we pulled didn't use the WS, and we didn't notice until later that they had either loss their weapon or never had one to begin with. This makes it imperative to get the position right asap so as not to waste a Mamool with a weapon.
IF for some reason they're being stingy, and you've ran out of mamools with weapon on the east side of the map, you'll have to kite some with weapons from the west entrance area. It's a pain, but do you want to win or not? ;D
So after rescuing the prisoner from either the 1st, 2nd, or 3rd door, you'll get about 20 seconds of dialogue, which will spawn some Runic something-or-other in the middle room with the pool in it. Next to it will be a chest. The person who opens the chest will get a ??? item, so you might want to decide beforehand who opens it (or maybe it's random, but our opener got the item). Also, it will drop some items. We got a hi potion+2 and reraiser in addition. Don't forget to have everyone lot or pass if you want them, because once you're out, the items are gone. When you're ready to exit, you must click on the Runic thingy to complete the mission and exit the map. If you do not click on it before the time is up, it'll end up in failure.