For this part, I’m assuming you followed the steps when exploring Club Hölle. You should be in the wardrobe area, so let’s continue.
Exit north past the wardrobe area then make a right. You should see a rainbow-painted shed with the “Staff Only” sign.
Inside, you’ll find the bartender disguise and emetic rat poison.
Head outside the shed and it’s very likely that you’ll spot Agent Swan dressed like he’s on vacation.
Important save: There are a few ways to eliminate him. The first is by subduing him and dragging his body to the corner (there’s also a coin here). Just cross your fingers that other NPCs won’t notice you.
When you reach the back corner, dump his body in the murky water. This also completes the “Spree Party” assassination challenge. Don’t forget to loot the keycard hacker to help you with exploration.
An alternative is to make your way past him until you see the Juice Bar.
Head to the left side and enter it (make sure you’re still dressed as a bartender).
Important save: Wait for Swan to get close. Then, serve him some juice. This completes the “Juiced Up” feats challenge.
He’s still hale and hearty, though, so you might want to add some flavor (namely emetic poison). Alternatively, put lethal poison if you have a vial already to get a poison kill.
Swan will start puking, so follow him to the nearby portalet. You can drown him here.
Note: The reason you’d want to save beforehand is because you might lack emetic poison (or lethal poison) later. It’s something you want to use against Agent Davenport, the dude we’ll take care of next.