To defeat Death very easily, just activate the Jupiter + Manticore DSS combo. His sickles cannot hit you, and by this point in the game you will probably never run out of MP because this combo will use it slower than you restore it.
Defeating Dragon Zombies (Twin Dragons)
To defeat the Dragon Zombies easily, whenever you get hurt, jump up to the platform by the door where you entered the room and activate the Jupiter + Mandragora DSS combo to heal yourself. A fireball may hit you every once in a while, but it is very rare.
More powerful shield and extra defense
Equip two items that increase your attack power, such as the Strength Armband and the Solders Fatigue (you should have +250 attack power), then use either the Fire or Ice Shield. Next, equip two items that increase your defense, such as the Defense Armband and the Strongest Armor. You will do extra damage with your shield and take minimal damage from enemies. Note: Your whip will be weak, so use your shield when attacking.
Shield with no MP consumption
1. Activate a shield (one using the Jupiter DSS card).
2. Enter a save location, and save your game, not turning off your shield.
3. Exit the save location, and your shield should still be going, with no consumption of your MP!
Shoot from the rear
Equip the Bear Ring (from Skeleton Medalist in a secret room on the left side of the shaft you fall down in the beginning of the game) and activate the last two DSS cards (Pluto and Black Dog). You will turn into a bear. While in bear form you can shoot from of your head. Hold Up and press Attack to shoot from your rear. Hold Down and press Attack to drop a bomb.
Spinning attack
When using the whip, hold down Attack to make it spin around in a circle. It will act like a shield, so you cannot get hit by small enemies around you. Note: You cannot move while doing this attack.
Start with All DSS Cards
Beat the game and enter 'FIREBALL' at the data screen.
Use any cards
After pressing R, but before the card takes effect, press Start. Go to the DSS Menu and pick any two cards.
Use DSS in Battle Arena
In the Battle Arena, equip a good DSS Summon Combo (for example, Uranus + Cockatrice). Press Start, select "Items", then "Use", and use a "Mind High" (not a "Mind Restore" -- however they can work, but you will need to be extremely fast). Immediately after exiting the pause menu, press the DSS button. When your character is done casting, press Down, Forward, Up, Attack. If done fast enough, you should have pulled off a summon in the Battle Arena.
Various cheats
Before starting a new game enter one of the following words as your name to have some alternate abilities (to confirm that you have entered a name correctly, check the option screen):
Magician- enter "FIREBALL" as your name. The word "Magician" should appear on the option screen. All cards will be available. You must have beaten the game ONE time to use this cheat.
Fighter- enter "GRADIUS" as your name. The word "FIGHTER" should appear on the option screen. Will be stronger and can sustain more damage. No cards will be available. You must have beaten the game TWO times to use this cheat.
Shooter- enter "CROSSBOW" as your name. The word "Crossbow" appears on the option screen. More hearts and weapon damage. The Homing Dagger weapon is now available. You must have beaten the game THREE times to use this cheat.
Thief- enter "DAGGER" as your name. You will start with Thief stats. You must have beaten the game FOUR times to use this cheat.
Transform into Alternate Monster:
Skeleton- Combine the Pluto Card with the Black Dog card and activate.
Bear- Combine the Pluto card with the Black Dog card and activate while wearing the bear ring.
Shining armor- Complete the Battle Arena and your prize will be the best armor in the game.
Action Cards - Top Row
Mercury - Bone Head
Venus - Slime
Jupiter - Heat Shade
Mars - Bloody Sword
Diana - Man Eater
Apollo - Arch Demon
Neptune - Ice Armor
Saturn - Fallen Angel (near the Battle Arena only)
Uranus - Scary Candle
Pluto - Trick Candle
Attribute Cards - Lower Row
Salamander - Bomb skeleton
Serpent - Earth Demon
Mandragoras - Axe Armor
Golem - Electric Skeleton
Cockarice - Stone Armor
Manticore - Thunder Demon
Griffon - Skeleton Athletes
Thunderbird - Were-Panther
Unicorn - White Armor (Battle Arena only)
Black Dog - Devil Armor (Battle Arena only)