Hard difficulty:
Complete single-player mode under the Easy or Normal difficulty setting to unlock the Hard difficulty setting.
How to defeat the final Boss:
At the start you will be in a room with a plasma cannon and an RPG. Grab both of them, then tap the activation panel to enter the Boss room. After a short cinematic, the fight will commence. Start by taking out your weapon of choice (other than the sniper or RPG) to destroy the four crystals around the room. Try to avoid the Boss’ attacks, as they will inflict a lot of damage. After destroying the crystals, fire at the center of the Boss where it fires the blue laser with the RPG. When it is destroyed it will keep doing the attacks that it performed when you entered the room. Destroy the four other crystals on the Boss with either the plasma cannon or RPG. After you destroy them you will have won the game.