Equip the Ultimate Weapon, then hold Y using the default control configuration. It takes some time for the weapon to get setup. The Critical Power attribute of your generator determines how quickly the weapon charges. As soon as you have begun equipping the weapon, a bar will appear at the top of the screen and quickly decrease. When that bar is empty, you will no longer be able to use the Ultimate Weapon. To fire the weapon, hold RT until a ring forms around your firing reticule. When that ring is complete, the Ultimate Weapon will fire. Depending on how quickly the weapon equips and how fast you can charge the weapon up, you might be able to fire the weapon again before it goes offline.