Once you've unlocked the end portal, you'll be able to reach a super flame powerup in the same room, which requires 14 spirits to unlock.
From here, you'll have to head through the powerup and then to one of two locations where the robot sharks are. One is the tunnel you took the submarine through, and the second is through a hole in the wall in the second room you reach in the level.
The sharks can eat and insta-kill you if you get too close, so shoot them from afar for safety.
Here's a clip if required.