If you have the time to spare, go to a place where only Specters attack you. Activate the Jupiter + Serpent combination. By the time you get to fighting Specter, you should have enough MP and INT to not have to worry about running out. Leave your system on AC power and leave it running all day in that area. Depending on your the level when you started, you can gain 20 to 30 levels.
When in the catacombs, go to the zombie location. Kill the zombies with a combo or something else. Wait for groups then defeat them to get to a high level.
Two conditions must be met for the following trick to work. First, you must not have killed the Dragon Zombies. Second, you must either have enough hearts to sustain a steady stream on axes (difficult), or have enough magic to sustain yourself in skeletal form for at least half a minute (easy). Start by saving the game. Step through the door into the arena. Do not move. You will remain on the ledge, with the door open. Move carefully to the end of the ramp. This is to avoid fireballs that could either kill you in skeletal form or knock you backwards or forwards. Either turn into skeletal form or begin lobbing axes. Eventually, you will kill the right zombie dragon. Exit the arena. The arena door will not close unless you fall from the door perch. Save to restore magic, if needed. Reenter the arena. The dragon will have revived. Kill it again. Repeat this as many times as needed. If you are below level 35, you will gain levels rapidly.
After you defeat Adremelech and destroy the Iron Maidens, go to the Underground Waterway (the one with the poisoned water). Then, go to the lower left wall of the first room of the Underground Waterway and whip the wall to open a secret passageway. Activate the Fire Sword DSS combo, if you have that card combination, and kill the Ice Shade in the secret room. Then, exit and re-enter the secret room. Keep doing the latter two steps until you have the desired amount of experience points.
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.
To defeat Adremelech (the goat head), make sure you are at level 25 or higher. When entering the room with Adremelech, get back up on the ledge where you entered and use the Wind Wave (Diana and Griffin DSS cards). Keep shooting at it, and in a few minutes the goat head will fall.
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 Dracula (Demon form) in fighter mode
Since you cannot use DSS in fighter mode, use the following strategy. When you appear in the Underworld, go to the left corner and duck. Hold [B] to use your Whip Shield until he turns around. Then, throw Boomerangs at the eye. He may send meteors or shoot lasers. Dodge the meteors and the stay behind Dracula. Repeat these moves. He will eventually start to flash. Jump on to the platform on the left. He will appear behind you. Use the Roc Ring to dodge him. His Eye will fly around. Dodge the bats and whip the Eye. Repeat until he is defeated.
Defeating Dracula (Demon form) in magician mode
You must be at levels 65 to 70. Equip the Neptune + Black Dog DSS combo. When he hurts you, it will not take as much life and will recover your life. When he is making his fire power, dodge it with the Roc Wing. Keep doing this until he turns into an eye. The rest of the battle will not be as difficult.
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.
An easier way to defeat the Dragon Zombies is to jump on the left platform where the locked door that leads to the trophy is located. From there you can not be hit by a fireball because you would be behind the dragon that spits them.
If you are in magician mode, use Pluto and black dog turn into skeleton. Keep throwing bones in the entrance or exit door, but turn back when they attack because you will die in one hit.
Enable the DSS combination Pluto + BlackDog. If you do not have these cards, then use the DSS glitch to do this trick. Once Nathan has activated DSS, he will transform into a skeleton. His only weapons are now only throwing bones, but randomly he will throw a giant bone that will inflict 9999 points of damage. This will effectively defeat many Bosses with one hit, including Cerberus, Iron Golem, Necromancer, Dragon Zombie and others, even normal enemies. Some stronger Bosses, such as Dracula, will not receive 9999 damage points, but the damage will be over 2000. You must play well, since Nathan will be defeated with only one hit in his skeleton form.
Summon the Manticore and Saturn card. This will cause a poison cloud. Once it touches an enemy of any kind, it will not let go until the monster is dead or you run out of MP.
Use DSS in the Battle Arena
In the Battle Arena, equip a good DSS Summon Combo (for example, Uranus + Cockatrice). In the pause menu, select "Items", then "Use", and use a "Mind High" (not a "Mind Restore" -- they can work, but you will need to be extremely fast). Immediately after exiting the pause menu, press your 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.
Enemies that drop DSS cards
The following is a list of enemies and locations that drop various DSS cards.
Action Cards Mimic Candle
Mercury: Bone Head in Catacomb.
Venus: Slime in Catacomb, Abyss Corridor (hidden room), and Audience Room.
Jupiter: Heat Shade in Machine Tower.
Mars: Bloody Sword in Chapel Tower.
Diana: Man Eater in Underground Gallery.
Apollo: Arch Demon in Underground Storage.
Neptune: Ice Armor in Underground Waterway.
Saturn: Fallen Angel near the Battle Arena with Dracula.
Uranus: Scary Candle in Iron Golem Boss Room (defeating Camilla optional).
Pluto: Trick Candle in Cerebus Boss Room (defeat Camilla first).
Salamander: Bomb skeleton in Catacomb.
Serpent: Earth Demon in Dash Boots Dungeon.
Mandragoras: Axe Armor in Abyss Corridor(in a hidden room), Audience Room.
Golem: Electric Skeleton in Audience Room before Necromancer.
Cockatrice: Stone Armor in Machine Tower.
Manticore: Thunder Demon in Machine Tower.
Griffin: Skeleton Athletes in Audience Room (hidden room way to the right).
Thunderbird: Were-Panther in Chapel Tower.
Unicorn: White Armor in Battle Arena with Dracula.
Black Dog: Devil Armor in Battle Arena with Dracula.
In the Skeleton Athlete room is a candle that looks different (after Camilla). Whip it and it will become a Mimic Candle.
Another place you can find the Mimic Candle is in the room you found Cerebus or the Iron Golem. This is easier than having to be distracted by the Athlete Skeletons. Note: Mimic Candle gives the Pluto Card and can only be found after you have defeated Camilla.
This trick requires the Roc Wing, The Jupiter + Unicorn DSS combo, and to be at the outer wall. Use the DSS combo and use the Roc Wing to jump up and down, killing Brain Floats. If done correctly, you will gain Mind Restores very quickly. It is possible to get forty Mind Restores in five minutes.
Go to the secret room where Lilith is located. Killing her and eventually you will get the Sage Robe, which is the best armor beside the Shining Armor. You can easily defeat Lilith by summoning the big electric bird. She also drops Mind Exs and also is worth 20,000 experience points. The secret room is located at the bottom right corner where you first meet the flying Succubus ladies. It is directly under the top door -- you have to hit the walls to find it.
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.
Combine Venus with Mandragoras and your luck will be boosted by 25%. This is useful when you need to find cards and items.
Defeat Iron Golem and go straight past him. Note: You must be level 15 or higher.
Combine Mercury and Cockatrice to make a Stone Whip.