In order to get this achievement, you need to find Paul alive in the secret MJ12 facility and speak with him. This can only be done if you help him fight off the UNATCO soldiers during your third and final visit to Hell's Kitchen. After killing enough of them, Paul will tell you he can handle himself from here and will make his way to Smuggler's. Don't worry about the army of UNATCO in the lobby at the Ton. You just need to help Paul kill the troops that attack his apartment and then find him in the medical labs in the next level.