That mentor is right. I got my 1st on my 4th or 5th normal difficulty crota raid. And my second came from a legendary heavy weapon engram that popped out of a level 6 dreg when doing a patrol on earth. I've seen em drop during Vault of Glass a lot too. Just do as many raids and the weekly strikes as poss and eventually you will get it. By "weekly's" I mean Nightfalls.