[Potential spoiler if you haven't beaten the story]
Just a little something to add to the other solutions posted here. This one is based on a random chance of an item drop. But I just wanted to post it here for people to be aware when they find it.
The best item I found to help out with this so far is a Puzzle Ring. The ring that I found has a property to randomly spawn an treasure goblin when hit. If you find one of these, make sure you wear it. I found my ring about half way through my Nightmare play through when I had just over 50 goblins found, and by the time I finished Nightmare I was about 15 away from getting this achievement.
One more thing, you may want to take the ring off for boss fights. It was a little more than alarming when a goblin suddenly appeared during the fight with
Hope this helps.