This level took me so many tries. I think I managed to complete it in about 3 hours, and yes that's a lot of restarting the mission and trying over and over again.
So for you not to spend so much time on this mission I'll just give you the most summarized walkthrough:

You'll only need a knife for fast kills if you don't get caught. Most of the kills are from the back.

1.First of wall - Ignore the outside, there's nothing, go to the restaurant.
2.Inside - ignore left side. Go straight and then right to find a guard in the bathrooms.
3.Murder him and the waiters in the bathrooms when they come in so you have the clothes ready for already 2 occasions.
4.Go to the bartender and take a leaflet to have access to go upstairs and talk to the prostitute/enter some rooms.
5.Find a part in the basement more likely a door to be protected by a lee hong's guard. Wait for another guard to pass, murder the guard, open the door, drag the dead guard as fast as you can so the others wouldn't notice. You'll find an agent there who you have to save.
NECESSARY!**** DO NOT SKIP THE DIALOGUE
Since I've tried it so many times i was skipping the dialogue over and over again. AND THE PROBLEM WAS NOT LISTENING WHERE IS THE REAL SAFE.
(at one point I thought the location of the safe will appear on the map once talked to the agent but that wasn't the case for me so just do not skip the dialogue.)

Yup, there are 4 safes, 1 is real and other are decoys. And they DIFFER every single game. To know all the four locations,
either spend your time walking around since the map is huge or just watch this video:
https://www.youtube.com/watch?v=IwIZNwXhDvU&t - it just shows which safe locations are there.
NOTE*: If you try to open the fake safe. All the guards will just come at you and that will be annoying, you can just instantly restart the mission.

6. BEFORE going to the safe you need safe code. So now go to the prostitute upstairs, just follow her and that's an easy part.
Now you have the location in your mind and the code.

7. Take the jade from the safe.
8. Change to the yellow waiter clothes ( idk if green works)
9.Go to the herbal shop owner near the restaurant in first floor.
10. exchange to poison, pick it up.
11. Go to the kitchen, insert poison in the soup.When the chef is as far away from you as possible.
12. bring the soup to the lee hong's main guard. He will die.
13. Run out the building as fast as possible to the nearest doors now. so the guards will not see the murder.(Although it's the most obvious one)
14. Change your clothes to the black suit so you can enter the building. Just kill someone lonely.
15. Enter the building.
16. Change clothes to lee hong's guard. I just went to the one i killed saving the guard.
17. Time for some mazes, but find you way up north in the headquarters.
18. There wil be many hong's guards.
19. Kill hong in his office since he is walking always. \
20. Grab the letter as fast as possible and run out the room so the guards would not suspect you.
21. Get to the boat.
22.YOU're DONE.

Hopefully I saved you some time doing this, and some nerves haha. Enjoy.