Sniper I, II & III are the easiest of the new achievements added to the game when the anniversary update came.
Start by going to Vlad's Shop and buy the new scope sniper rifle. The best way complete all 3 achievements fast is simply to wait until an attack on your border occur as you will need 20 camera effect kills. These kills are easy to trigger. Going for headshot on long distance enemies whilst holding your breath seem to trigger the effect 90% of the time for me.
NOTICE:
You do not need to start a new savefile to complete these achievements. I am certain most if not all new achievements can be obtained in inspector mode. I am currenly as of writing this little guide doing NPC stories in inspector too.