Instructions
Collect the Silver Arrow from the basement level of Gannon's dungeon, then proceed to Gannon's lair.
Press "A" to dismiss Gannon's opening message to you, at which point he will become invisible. Stand in the middle of the screen and dodge Gannon's energy bolts while slashing with your sword.
Slash until the invisible Gannon blunders into your sword's range, which will make him freeze and flash blue.
Move away from Gannon while he is frozen to avoid his next attack when the freeze wears off. Dodge it and return to the middle of the screen. Rresume slashing at him and keep dodging his energy bolts.
Hit Gannon three straight times. When he flashes red, equip your bow and select the Silver Arrow.
Fire a Silver Arrow at Gannon before he stops flashing. This is the only weapon that can defeat him. One shot from the Silver Arrow will finish Gannon and end the game.