This description is wrong, it is actually 'kill 20 enemy snipers in one playthrough'. This will easily come with main game progress as there are way more than 20 in the game total. If you want to get this fast, you can find an outpost with some snipers, kill them, fast travel away and come back and they will have respawned. You can also kill them, die and restart at the checkpoint to also have them respawn.