Use endless mode for this. Find a good single entrance corner to hide at so all the enemies come to one place. A katana is a great starting weapon. After a few minutes, you'll find a couple guys with rifles. You need to get a hundred kills with rifles to get the achievement. Try to line up multiple guys to hit with one group of shots. I'd recommend the bridge map, since there's an excellent single entrance hideout right behind where you spawn if you spawn at the place where 1 guy stands next to a katana. If you've completed the story, you'll likely already have significant progress for this achievement. Once you reach 100 rifle kills, you'll unlock this achievement.