Try to get a black security guard suit and put your poison in the coffee on the upper restaurant ;) The best moment is when your target is looking out of the window inside the restaurant. It's a bit tricky but it should work.

And how to get that black suit?

There's one guard patrolling from the 3rd to the 2nd floor and back. Also there's another one on the 2nd floor flirting with a secretary. Anesthetize booth and get the suit.

Thank you!

Ummm, so there's a knife in the level and the poison has a use :grin:

***SPOILER***
I've just finished the level by walking into the third floor room where there's a diplomat, a secretary and a black-suit guard. Used chloroform on secretary, kill guard, chloroform diplomat. Move bodies into diplomat office. Wait for target and shoot in the head.

Took about 20 retries to get the correct timing to be able to hide all bodies before the door to the main corridor gets open when target enters.

I'll try again the mission and use the poison.