I don't know if I believe the 'kill more prisoners to get the 5th to spawn', cause I've done the mission twice, and after spending a couple of hours of searching and killing, there was still no 5th prisoner. We killed a great number of goblins, and still no 5th prisoner. There needs to be a counter or something to know how you're progressing, if that is what it takes. But, I do understand you can't just have the prisoner up, or people would just kamikaze hail all the prisoners and win the mission.
Also, I think the respawn here is really fast, not that there should be any respawn at all in my opinion. I really hate this mission, and I say that cause I've spent hours in it and have not won it.
And has anyone beat the Guk monster mission where you kill the NPCs at the camps? The one given out by the NPC outside of Upper Guk in Innothule. I haven't tried this since the first time (again wasted hours with little results, even after we 'got it'). I imagine the common reward is plate gauntlets, since I haven't seen them as the reward for other missions.