Unlock Hard Mode:
Beat the game once on "Normal Mode" in order to unlock the extraordinarily difficult "Hard Mode."
Purpose of the doll:
The doll in Prairie's room isn't just there for decoration or to attack just for fun. If you continually strike the doll with attacks, in time it will drop a "W Can." The "W Can" or Weapon Can refills all of the weapon energy for all of the Live Metals. The doll will only drop it if you don't already have one. So return to the doll after you use the W Can to get another one. Cutting the doll from its string makes obtaining the item much easier.
Keep Model X:
Complete the game with both Vent and Aile on Normal or Hard difficulties and save their "Cleared" save files. Now, when you play through a New Game again as either character, you'll keep Model X after acquiring Model ZX.
Beat Serpent Fast:
Get 3 Sub-Tanks, and fill them up, you'll need them for the 8 bosses that you re-fight. Once you get to the top where Serpent is, do NOT enter the door that leads to him, instead, dash back and forth killing the 3 guard Mavericks to refill your Sub-Tanks. Now his first form is easy as hell, but to do it so you have no hits at all, use Model Phantom X PX, Wall Climb up to the top of the screen and stay up there firing, if he gets too close, dash to the other side, firing at him on the way.
His second form is best taken on with Model Zero X ZX, which you change to already via the cutscene. Just keep slashing at him while slashing down his attacks, eventually you will get to the core with Zero's face. Slash at the core as fast as you can, because you can hit it from anywhere.