We were fighting inbetween the two gates there, taking out the mobs at the gate, the patrols and the mobs in the smaller camps at the side. We were working on our second camp when we notoced that the boss, Nerzus if I remember correctly, had spawned. He's standing at the entrance of the big tent. I don't know however if the "area" is larger and maybe others were fighing in the "area" as well - nor can I remember how many undead we killed. I'd suggest to make some pulls and keep doing that until he shows up.
Carannar & Alatir ~ minstrel and guardian of Invicta