Hail Bugluk to turn him attackable (level 77^^ heroic).
When you get him to about 30% health, he will run away.
Suspend any mercenary you are using before attacking. Bugluk will not run away, instead remaining engaged to the merc. Otherwise, the quest updates normally, and you can evac out to break the encounter or wait for the merc to die.
Return to Nesbit at U.J.L. 1.0. You will permanently lose the Boarfiend Costume when you complete the quest.