From Zelda Legends Wiki
Guardian of the Great Palace and second-to-last boss of the game. It can only be defeated by casting the Thunder spell to reveal its weak point. After casting Thunder, Thunderbird will change color from red to blue. His only weak point thereafter are sword strikes to his face and after every hit he will speed up in both his movement and emitting fireballs at Link. To lessen the damage from Thunderbird's fiery assault use the spells Shield and Life when your health is lowered. Thunderbird is probably the most difficult boss out of all the 2D Zelda games.