Tip: Turning Aim Assist "On" will make sniping easier.
In the first mission, in the beginning, when you go up a ladder and then crouch under a vent, you can pick up a sniper rifle. You will have more than enough ammo to kill 20 enemies. Just keep using the sniper and go for head shots or torso shots. Just stand far back behind cover and keep shooting enemies. They will keep respawning for a while. You should have more than enough enemies to kill in the first mission. If you don't get the achievement in the first mission, you will have more sniper ammo in the second mission.