One of the hardest enemy in the demo is the guy on the roof, so be careful not to step to the open area shown red on the picture left, or he can easily snipe you down.

To kill him, the best way is to first clear out the main building, then crawl to the window, near the sliding door (shown on the picture right) Then get up to prone position, and shoot the guy (remember to press tab to change level) Be sure to leave least 2 APs to get down, so he can't shoot you back.

You can also shoot the guy from behind the boxes, but to get there you have to come from the sliding door.