super easy to obtain near beginning:
As mentioned in the comments below once you have your camera open you can just tap on repeat and it will keep taking photos over and over - this no longer works
UPDATE - they improved the photo mode since i posted this guide
you will now need to:
Press
select your camera
press (to capture)
To take multi photos just press then on repeat repeat 50 times
once you have done it 50 times press
go to dreamlight then scroll down to friendship
Here you can see how many photos you have taken (you need to collect your rewards in order for the achievement to pop)