Thunderbird

The Thunderbird (ボルバ, Boruba) is the primary guardian of the Great Palace and the second to last boss in The Adventure of Link. It is a bird-like creature that was placed there as one of the many tests prepared for the hero seeking to acquire the Triforce of Courage by the ancient King of Hyrule that once ruled over the entire kingdom with the united Triforce. In order to defeat it, Link must first cast the Thunder Spell to expose the boss's weak point, its face. The Thunderbird's only attack is throwing fireballs down at Link. The more the Thunderbird gets hit, the more fireballs it will throw while increasing its speed. After defeating the Thunderbird, Link will proceed to the chamber where the Triforce of Courage is kept by an old man, only to face and battle Dark Link before he can claim the relic.

The Adventure of Link
In the backstory of The Adventure of Link, it is said that a wise King of Hyrule once ruled over the land of Hyrule and used the united Triforce to govern the land during a golden age. However, as his death neared, the King decided that no worthy person existed then that would be able to successfully wield the Triforce without producing evils for Hyrule. In light of this, he broke the Triforce into its three component fragments and left the Triforce of Power and the Triforce of Wisdom within the kingdom. The Triforce of Courage however shared a different fate: the King spirited it away to the Great Palace atop the Valley of Death and concealed it deep in the bowels of the palace. The King left a primary guardian in place (as well as a secondary guardian in the form of the shadow of the one entering the palace) to guard the pathway to the chamber of the Triforce of Courage, a powerful and agile beast known as the mighty Thunderbird. For an age, the Triforce of Courage remained hidden away within the Great Palace, with its final resting place being guarded by the Thunderbird, who awaited the coming of one seeking the third Triforce piece.

Many long generations after the King concealed the Triforce of Courage within the Great Palace and after his daughter, Princess Zelda, was cast into an enchanted slumber by an evil magician for her refusal to divulge the final Triforce piece's resting place, the young hero known as Link, who had vanquished Ganon a few seasons earlier in The Legend of Zelda, realized that upon the arrival of his sixteenth birthday, a strange mark revealed to be the crest of the Triforce appeared on the back of his hand. This marked him as the hero sought by the ancient King of Hyrule to retrieve the Triforce of Courage and reunite it with its two compatriots. Link sought to do so in order to awaken the ancient Princess Zelda from her unnatural slumber, and after a long journey restoring six magic crystals to statues in six other palaces across Hyrule, he succeeded in lifting the barrier left across the Valley of Death by the King, granting him access to the Great Palace. After fighting his way through the immense palace, Link came upon the chamber lying just beyond the chamber of the Triforce of Courage. Little did he realize that this was the chamber of the Thunderbird, who soon appeared and attacked him from the air. Utilizing powerful magic spells he'd learned on his journey as well as a good deal of courage, Link was able to discover the Thunderbird's weak point, its concealed face, and was able to exploit this weakness, eventually vanquishing the ancient creature once and for all to clear the way to the final chamber of the palace, where he would face one final challenge before being able to take the Triforce of Courage.

Strategy
When Link enters the chamber of the Thunderbird, it will not be anywhere to be seen at first. However, after a few moments, it will swoop down upon Link from above and proceed to attack him with fireballs. It will have a red color to it and will at first be invulnerable to attacks, leaving it invincible at first. To reveal its weakness, its face, Link must cast the Thunder Spell, which he can learn from an old man in the Old Town of Kasuto after attaining the maximum level possible for his Magic Meter. Casting this spell will cause the Thunderbird's color to change from red to blue and will cause its face to appear, leaving it vulnerable to sword attack. Link can then slash the beast's face with his sword, inflicting damage upon the palace guardian. In response, the Thunderbird will quicken its pace of throwing fireballs, which can only be repelled with the Reflect Spell, at Link. Once it has taken enough damage to its face, the Thunderbird will disintegrate, leaving behind the key to the palace's final chamber, where Link will face his final test to prove his worthiness of possessing the Triforce of Courage.

Trivia

 * His name could be a reference to the mythical Thunderbird, a giant bird who kidnaps children. Note that the Helmaroc King from The Wind Waker acts like the mythical Thunderbird (he kidnaps female children).
 * Like Gohdan in the Tower of the Gods in The Wind Waker, the Thunderbird is the guardian placed to test the courage of potential heroes.