You must choose the mission Jackson: The Truth, else you will have the Killer on the level (and he'll ruin this plan.)

This is the principe of this guide, break the last window.
You need the hangun.
AIM like this and shoot.

All the next steps need to be done before the sniper appear.

There is a guard looking at you on the 4th floor.

You can Trigger the alarm to make him turn his head.

Go trought the window, go upstairs, go downstairs (hammer down key ^^' ) and start climbing.

or try (credits to [NL] Noldemor )
Just when youre right under him jump right upwards with a really low angle to the right.

Climb as fast as possible. run into the Gessler office. To avoid to be shoot take the handgun when you're back on your feet. You can now punch or shoot Gessler. Go to the subway by jumping through the windows.

Thanks for reading, hope you enjoyed.
All comments are welcome. Don't forget to vote.
PS: try my map