It's best to combine this side objective with retrieving the report of the assassinated sniper. Once you have the report, exit the tower and approach the bridge (M2.5): you will be able to spot a window, at which the operator spends most of his time (M2.2). He will leave his building from time to time, so if you don't spot him right away, just wait for a while. You won't have any problems with recognizing him - he has a large radio on his back.
The wind should not cause much of a problems so taking him out should be easy (about 120m). You could also come a little closer and adjust the weapon to 100m.