This is pretty straightforward, kill 100 people with a sniper rifle. When getting into a match make sure to hit RBumper until your class is M(for Marksman) if it's not your default as this will give you bonuses with sniper rifles, which are listed as Marksman when choosing a weapon. This achievement is surprisingly easy. GRAW 2 has relatively large maps so sniping is always a viable option on all but a few maps such as Lagoon and Fort where the terrain is made for short range combat. It also helps that sniper rifles work well as close range weapons. You can tap the trigger rapidly while not looking through the scope and the rifle will act like a super powered pistol with a larger clip.
The best rifle for long range combat is the M107 as it is extremely accurate and kills with one shot no matter where you shoot them. I've only once seen someone survive a M107 shot. If you simply don't have the patience to snipe grab either the SRD 25 or the 30 round sniper rifle that looks like an M4 and use it like an assault rifle on semi auto. It works well, too well in my opinion.
Be patient and you'll get this.