While Nomad's answer will help you complete the quest, it doesn't answer your question as to where he is. You need to go to Upper Marten's Bluff via the elevator in Lower Marten's Bluff. Now you will need to lure the assassin into the trap Z'ant was talking about.For more details on how to do that, highlight the following: Once you get to UMB, you will find yourself in a room with metal walls on two side and glass doors on two sides, only one of which is (or will) open to you. Go out it and to your right through another door. There's a lever in that room next to a big machine (not the computer panel). Push the lever, which will drop a dummy rapax down in the room you were in originally. Use the computer panel now to open the other door in the room and to learn how to operate the trap. Go back to that room and use the potion you bought from Crock on the dummy. I think you can figure out the rest from there without help. Good luck!