The final battle with the overlord can be very difficult. However, there is one trick to beating him. Equip a gun type that can shot at a down-left angle. Then simply fly right in front of his forehead, the only place none of his attacks can reach.
New Game +
To unlock New Game +, beat the game and wait through the credits. At the end, the game will prompt you to save. Do so, and the next time you start the game the completed save file will have a gun data icon by it. New Game + allows you to replay the game with all of your old gun data, and gives you access to several new endings.
No Random Battles
Once you obtain the Girl Wings, you can use them to avoid random battles. You cannot enter a random battle while hovering. So if you repeatedly use the Girl Wings as a means of travel instead of walking normally, you'll never be summoned to play a random level.
Psyme endings
In the final battle with psyme she is in a battle worm and you have to fight her. If you only shoot out the three cannons on top she remains alive to the end. But if you shoot out the eyes on the bottom of the screen she dies, and is out of the last cut scene.
Scarlets Endings
Whether Scarlet appears in the ending or not depends if you pick up the Virus while on earth the second time or not. You can find it in the room above the one where Zart and Tierney were working. You cannot return here after you have left, so decide what ending you want here.
Unlock Sigma Ship
Defeat the game once to unlock the Sigma Ship in random encounters.
Final Boss trick...