Agent Chamberlin is patrolling level -2 of the Berlin nightclub. He goes back and forth the dance floor and the security room (which is where you can wipe camera footage in case you were spotted).
You can sabotage the wires and water dispenser to electrocute him, or you can distract him so he heads to the bathroom where he can be silently assassinated.
There’s a related feats challenge called “Whiteout” and it requires you to envelop him in a cloud of cocaine. While the cocaine brick can be found in the Biker Hangout where Agent Lowenthal is, I haven’t figured out how to actually trigger this challenge for Agent Chamberlin. It seems to hint at using a ventilation shaft or control panel.
Anyway, let’s talk about Agent Lowenthal and the pizza delivery guy.