Octogon

In Oracle of Ages, Octogon (オクタゴン, Okutagon) acts as the Boss of the Mermaid's Cave. It can spit rocks like it's relatives, the Octoroks and does so when he is above the water, which gives Link the ability to use his Seed Shooter. It bears a resemblance to Big Octo, who appears in Ocarina of Time.

To defeat this creature, Link stood on the only dry part of the room, tossing either pots or bombs into a hole on Octogon's head whenever he could, and attacking him with his sword if he could manage it. After a while, Octogon would go underwater. Following him using the Mermaid Suit, Link quickly discovered the Octogon was far more aggressive underwater. Despite its aggressiveness, the Octogon finally went the way of the monsters before it, Link overpowering it. Without wasting time, Link then proceeded to take the Lonely Peak.

Strategy
Octogon is one of the easier bosses in Oracle of Ages. When Octogon is swimming on the surface, Link would throw the two pots, as well as bombs, into its head. When it turned to fire a projectile at him, Link could slash its vulnerable flesh with its sword. When it fled underwater, Link could pursue it and continue to attack its flesh.

Trivia

 * Hardhat Beetle are relatives of the Octoroks sometimes referred to as Octogons.