Kill LUX, if you use the key you get from his side quest to enter the back room and the artifact he will initiate a fight when you go back up unless you start your own ambush before the dialog ends. which is recommended. That way you can pick off half of his team before dealing with him. making the fight much easier. Note he can mind control both dead allies (resurrect) and your team members. so keep his actions to a minimum. Killing him will make the mind control go away.