Kill 100 Treasure Goblins.

Treasure Goblins (Pygmies) are fat little, money bag carrying goblins that will drop coins whenever you hit them as they run away from you. If you can successfully chase them down and finish them off before they open a portal and escape, they will drop a lot of gold and items. Repeat this process 99 more times and the achievement will unlock.

This may or may not be a tip, but I find Treasure Goblins far more often while playing in Hardcore mode, and those kills will carry over to your primary playthroughs.