If you missed the spot in mission 2 like I did, another easy location for this is right at the start of Mission 16: Eye of the Storm. Should take you no more than 1-2 mins.
Right after you get out of the water, pass the guy at the railing to your left and straight ahead of you you will see two cell guys chatting. One will be leaning up against the wall. These guys are perfectly stationary and you have plenty of time to get lined up while in stealth.
I would NOT recommend trying this while doing Stealth Assassin, although when I did it they went down instantly and I didn't get any attention whatsoever.