This is what I did and got it on my first shot..
Make them meet obviously the organ then head to the roof of your ICA hideout.
(It takes a LONG time till they meet at the top of bell tower)
But in the meantime, take out the two (KO) NPC's on the roof. One is sitting in the chair and another on Rocco's balcony. Once they are gone, get your sniper rifle and get in position.
(You will still at this point be waiting a good 5 minutes lol)
But once they meet at the bell tower and you have them in your sights, give it a few seconds and then snipe Marco. As long as nobody sees you rife the rifle obviously, you will be fine. Then exit.
This is exactly what I did and it unlocked no problems.