Instructions
Start the game's campaign story mode. Select a difficulty of "Normal" or higher.
Look for a hidden skull in each level. These small human skulls often blend in well with the environment.
Pick up the skull by pressing the "RB" button if you are using the default controls, or the "B" button if you are using the alternative control setup.
Quit the level, if you wish, by pausing and selecting "Save and Quit." Do this only if you have already completed all the levels and have the ability to select a level from the main menu. If you cannot do this, drop the skull and finish the level.
Go to the next level and find another skull. Continue doing this until you have collected all 13 skulls in the game. When this is done, the final piece of the Hayabusa Armor, the helmet, is unlocked.