I would continue to hunt ALL the undead around the mob your supposed to kill. It will spawn eventually.... or it should.
You never know, it might not be the raiders your supposed to kill, maby its the other skeletons beside it.
I know I had to kill mobs for a quest for a long time before the quest mob appeared (another quest). Another quest I had they had a bad translation, and mobs that wernt supposed to give me quest items did.
Stratics is pretty good with their quests. Im sure plenty of people have followed that quest before. Dont worry , the quest mobs will spawn.
I did that quest also, and I ended up killing about 100 or so mobs before he spawned. I was quite frustrated. Just keep killing them until he appears. It will happen eventually, although you might be wearing A Grade by then (j/k)
When you kill them...I was at lvl 39 and got about maybe 2% or more on my xp bar before he spawned of course I scrwd up and pwnd him too fast to use the sword and had to do it again...but it happend the same a little over 2% xp gain and he spawned.
Seems like it's horrible random on some quests, I think it took 10% for me at that part. Or like the quest with Nahir, once I got the quest item on the first try, other times it took 10+ kills.