Techniques: Rapid Fire Missiles
Switch to the Power Beam to fire Missiles much faster than you would normally. If you press A on the controller after firing a Missile with the Power Beam active, you can close the Missile Launcher and fire another Missile almost immediately. Repeat this to quickly dish out huge amounts of damage.
Tzyr adds: Take your thumb and point it roughly so that your nail is 90 degrees to the buttons. Now with just your nail, you are just flicking Y really fast. You will hit the part of the controller between Y and A, but because your thumb is fat enough, it will slightly hit A, which is all you need to reset the gun. So the faster you do it, the faster you will fire, but know you can only have 3 missiles in the air at once, so the closer you are to the target, the better.
Alternatively, you can simply switch to the Power Beam, then hold up on the C-Stick to rapid fire Missiles.
Examples of Rapid Fire Missiles include quickly killing Meta Ridley's ground form.