The best place I've found to do this is in the second campaign-Quest 4 "Tower of the Necromancer"-location 3 "The Shadow Arsenal" where you have to escape through the door to progress.
Every time you defeat an enemy more goblins spawn (which counts towards the achievement.) Knock out all other hazards until only the door remains, this ensures the maximum goblins can spawn. But only spawn as much as you can handle or you're going to have a bad time.
It doesn't matter if you fail the quest either, they all count.
If you have a good team put together you can rack up to 50+ kills a round before failing the quest.
Just rinse repeat. Pop