Instructions
Press "Start" to open your Inventory when you see a Beamos. Equip the Mirror Shield if you have one, and assign the bow to one of the C-Direction buttons. Unpause the game.
Ready your bow by tapping the C-direction button it's assigned to, while circle strafing the Beamos. Lock onto the Beamos by holding the "L" button, and roll when necessary to avoid its attacks. Be careful when you decide to fire, as you'll need a second or two to ready an arrow for shooting. Three arrows kill the Beamos.
Equip the Mirror Shield, if you have that item. Attract the attention of the Beamos, then wait for the sound that indicates that it is ready to fire the beam. Hold up your shield with the "R" button to reflect the beam to destroy Beamos. This can be more useful, as it conserves arrows that you will likely need to defeat faster enemies.