Restart a battle
Pause the game if a defeat cannot be avoided during a battle. Select "Equip" and resume the game at the start of the battle, with all energy restored.
Get Mar onto the ship
To get Mar on the ship, go to Sephy and Zeus. When you have a choice to let them join you, say no. Go back to the cabin area and you'll see Mar.
Regain health
If you are running low on health, just shoot or attack the Electric elementals (small wavy line of electricity). They will go away and refill your health. They will reappear soon after that, allowing you to refill your health as many times needed.
Defeating Gaia
Equip the Hand of Pyro and Bite of Lightning. Attack and destroy the two arms, then the white-orb. Block all the attacks Gaia does. Use Bite of Lightning to easily destroy the little robots Gaia throws at you. Do not call the Elemental Summon; you will not have enough time. When only the head remains, shoot Hand of Pyros at Gaia until he dies. Remember to block all attacks.