the mission explicitly say u need kill the harbringer.and keep the npc alive.even if u flag heroes/hench with him and go alone the risk the groups get him are big if left alone.
+ u need the ligh droped by other harbringer to weaken the boss.so erm.
Masters would be impossible to run, as you have to kill the Margonites on the stairs along with the Harbinger at the top. Running the mission MIGHT be possible, but very difficult. All Harbingers except the last can be ignored, but the patrols of Margonites would probably get to and kill the NPC before you finished the run. Not sure it would work, but consider trying this:
Run to the Harbinger near the base of the stairs for the bonus, and kill it. This will spawn a spirit to create a new Light to use. Cast Recall on the spirit, grab the light, and run to the last Harbinger and drop Light to damage it. Cancel Recall, grab new light, reset Recall again, and go drop Light at Harbinger again. Repeat this one more time, and you would have finished the mission (3 Lights will kill final Harbinger). I doubt you'd be able to do it before the NPC is killed though.
Running the mission is quite possible.
You take Koss + Tahlkora + Dunkoro and flag them around the NPC. Tell the people you're running to wait there and guard him as well. They might not be able to defeat the margonites but they'll easily be able to keep the NPC alive.