In the Asgard hub world there is a mission you can get from Malekith that requires you to defeat 20 enemies for a gold brick. This achievement should pop after you beat the last enemy, if it doesn't simply back out of the hub world and try again.
You can do this on any Hub world mission that has enemies to fight, this was just one that I remembered having exactly 20 enemies and being easy to find
EDIT: As mentioned by Atedeate, you can get this achievement is by replaying the Daredevil free-play mission in Manhattan. It is located in the Dockyard area.