When you arrive in Shanghai, head towards the Hung Hua Hotel. Once inside, head up to the 4th floor and head to the first open door up ahead to see a guy arguing with a female. Once the guy has left, go over and talk to Mei Suen. Accept her mission to find and save her friend Ning and save her from the gang.
Once you arrive to your objective, remember to take everyone out with none lethal takedowns and weapons then open the garage door to find Ning. Speak to her, and she'll flee and then return to Mei to hand in the mission.
You'll now be asked to kill Diamond Chan but you have to make it look like an accident. So either push him off the roofs to make it look like suicide or plant some drugs. Accept this task and head up to his apartment on the roofs. Before you open the door or take him out, save your game. Now open the door and when Chan is not looking knock him out.
From here, drag his body outside, and push it of the edge so it falls to the ground below. 'The Fall' achievement should now unlock. Reload your saved file but this time after knocking him out, plant the drugs on the small table beside him. Now head back to Mei and hand in the mission to unlock the 'Rotten Business' achievement. You don't actually need to plant the drugs to unlock this achievement, you can do the other option, but to be safe and not void the 'Pacifist' achievement, it's best to plant the drugs.