So I don't want to make this super detailed because you need to proceed on this path to continue the story, and it's fun to experience it on your own. Once you find the cyclops gem, before picking it up, check the room on the other end of the bridge. Kill the two skeleton creatures and pick up the Krell Hammer for the achievement.
If you do pick up the cyclops gem right away you will hear the door to that room shut Just place the gem back on the pedestal to reopen the door and grab the hammer.