***WARNING: MILD PLOT SPOILZ***
To get this achievement, you must be playing co-op as Lynch. It takes place in the chapter "Exchange" (the 6th one), which begins with Kane walking through the park to the pagoda to meet Retomoto while Lynch is up on the overpass guarding Retomoto's daughter. After a cutscene where Kane speaks to Retomoto on the phone and Lynch flips out and shoots the hostage, the action begins. There are two snipers who are part of Retomoto's ambush for you... the achievement is for Lynch, who has a sniper rifle, to kill both of them within 15 seconds of the end of the cutscene.
To fire the sniper rifle, hold LT to sight and then while holding press UP on the D-pad to zoom in. If we imagine the circle of the overpass as a clock face, where Lynch's position on the dial is 6 o'clock, then the snipers are located at 9 o'clock and roughly 2 o'clock (this one is very close to a van which makes him much easier to spot than the other one). Have your buddy playing Kane just crouch down in the pagoda hiding from damage so he doesn't ruin the achievement by dying. No need to try for headshots while sniping them, a shot to the torso seems to kill them 90% of the time and you risk wasting time and/or missing by going for the head.