Sharks are the large void monsters. If they approach an object they will pick it up and throw it at you. You need to throw a barrel at their barrel in midair with the Phantom Grapb
skill to obtain this.
will slow time, giving you more than enough time to line everything up. The difficult part is getting a shark to pick up a barrel while you are within range of picking up your own barrel.
After obtaining the third Omensight choose the Emperor. Follow the linear level and just near the end of the level there is a section that spawns 2 sharks, several barrels and is immediately after a checkpoint. If you mess up, you can jump into the pit and try again with 20 seconds.
Here's a video that shows my succesful attempt. I failed with the first set of barrels, but with the second I had all day to line up the shot once the shark picked up its own barrel.