You are a sniper on a rooftop. Enemies pop up in the lit windows and you have a couple of seconds to take each one down before they shoot you. Some windows show civilians in blue: do not shoot them.
On a phone: drag your finger to move the scope (it sits a little above your finger so you can see it) and tap the big red FIRE button.
On a computer: move the mouse and click to fire (space also works). R reloads.
The scope sways a little like a real rifle, so time your shot. A headshot (the head of the figure) scores 200, a body shot 100. You have 8 bullets per clip, 3 lives, and you win by taking down 12 enemies.