This achievement will just happen over time as you're playing through the dungeons and leveling up or doing quests.
You just have to defeat 100 goblins which only takes about 45mins from the start of the game.
If you're looking to "farm" early you can do this at "Lorien Hollow" as theres quite a bit of goblins there.
If you vote down, leave a message and ill happily fix the solution.