Poseidon is a large bearded man with a mermaid's tail. He attacks by smashing his trident into the ground and summoning Blow Fish.
Poseidon is a relatively large boss, consuming a lot of the room. Each Blow Fish Explodes once coming in contact with player or walls. Avoid Walls to avoid splash damage and circle Poseidon to ensure greater survivability from his attacks.
If you stay directly under Poseidon his attacks will go right over your head, making this an extremely easy boss fight.