Instructions
Play through the game until you find a ballista, which looks like a large mounted crossbow. Press "Up" on the directional pad to select the ballista arrows as the ammunition for this device. There will be a pile of this ammunition next to any ballista that you find.
Press the "+" button to jump up onto the ballista. Use the control stick to aim the weapon.
Press the "A" button to attack using the ballista. You can only attack for as long as you have ammunition, so check in the boxes next to the device if you need more. Press the "B" button when you want to leave the ballista.