Lurbuk lives in the city of Morthal, so let's fast travel there. Like Nazir stated when we took this job, this is an EASY kill. Lurbuk stays at the inn in Morthal, so wait until night (4 AM or so) when he'll be asleep and enter the inn.
Lurbuk will be on the first room on the left, so enter there and then sneak to make sure you're hidden. From there it's as simple as a sneak attack (ensuring your weapon is strong enough for a 1-hit kill) and he's dead. Loot the body and get out of there. Now you can either do the LAST assassination or you can report to Nazir (I suggest doing the last one; be sure to save!). This contract is worth 500 Gold .