After completing the game, return to Concorndia and check the job board between Moxxi's and the Meriff's place. One of the available missions will be to defeat a raid boss--The invincible Empyrean Sentinel. Return to where you found the final boss. You will need 20 moonstones to enter.
The Invincible Empyrean Sentinel has the same modes and attacks as the final boss. He is much tougher, but unlike other raid bosses, he is stuck at level 28. I easily solo-ed him at level 35 with weapons from the claptrap DLC. If you don't have great weapons, you might want to wait for level 40.