Aiming

The first method it's by using a scope, most rifles and some another weapons have the ability of equipping a scope, this scope can increase your aim when trying to hit a target over a long distances.

The second method it's holding right mouse buttom, by holding the right buttom of the mouse your soldier it's going to bring his rifle closer to his eyes making it easier for you to aim.

The third and last method it's actually a continuation of the second method: while holding right mouse button, hold " SHIFT " at the same time, this will make your soldier hold his breath making aiming over a long distance easier. (notice: based on the difficulty you're playing on your soldier can only hold their breaths for so long)