This boss is fought at the end of the "Embrace" playthrough (using all 7 Shards to corrupt every ability), and is quite challenging at first.
As I said earlier though, unlocking the fight is simple - play through the game and corrupt all 7 abilities.
There's 1 hidden Elder Shard, each fragment is located in each area of the game.
Here are the locations:
*** Spoiler - click to reveal ***Fragment 1 - In the Bastion (path to the first area's boss, Dominion) there's a section at the top with 2 constant lasers blocking the floor and ceiling. Use the Cannon to propel yourself across the room to get the fragment.
Fragment 2 - In the big drop room of the second area (the one with the door at the bottom), there's a section on the right that has the fragment. Hug the wall as you fall, perhaps using your double jump and/or cannon to stay against the side. You'll eventually fall into a small alcove that has the fragment on it.
Fragment 3 - The location has been changed in an update to the game, but it should be in the bottom-right of the map. I advise getting the Treasure Hunter upgrade to help locate it.