Dimension Slip accessory

When obtaining the Bark Of Dryaid in Nolsoft Swamp, keep it. Then, when obtaining the Transformation /Creation Gem, change it to get the "Dimension Slip" accessory.

Hard mode

In Dark Tower of Zelva keep the Secrets of Resurrection artifact and then change it to an Orihalcon. Then, change the Orihalcon to get the Creation Jewel. Use this to obtain the Glare Sword, Icicle Sword, and Crimson Edge spear from Broken Armor, Book on Everlasting Life and Broken Spear respectively.

Seraphic Gate

Save in the final save point in the game, before the final Boss. When you reset the game, there will be a new option called "Seraphic Gate." It is a bonus dungeon that is separate from the game.

Best ending

To get the best ending, you should keep your Seal Rating below 37. Factors that affect it include:

Transferring a character: +12
Recruiting a character: -2
Not wearing Nibelungen Ring during Sacred Phase: -2
Going to weeping Lily Valley, Valkyrie's Graveyard: -15
Meeting Lucian in Gerabellum before you recruit him: -15
Meeting Brahms at his Castle: -15
Meet and defeat Lezard Valeth at Chapter 4: -15
Recruiting Mystina: -15 (only if you defeat Lezard at C.4)
Recruiting Lucian: -20

Short elevator rides

When in the Tower of Lezard Valeth, you can shorten the time the elevator moves from point A to B by opening and then closing the menu or map screens. You can avoid battling the monsters that ride the elevators by using this trick.

Gram

If you are lucky and get another Orihalcon and you have the creation jewel (not gem), transmute the Orihalcon to Gram, a powerful sword that cannot be overpowered until you get with your fight with Loki in Ending A.

Flame Jewel locations

You need Flame Jewels to access special rooms in the Seraphic Gate. In them, you can find the best equipment and the secret characters. There are eight special rooms, meaning you need eight Flame Jewels. They can be found in the following locations. Note: Flame Jewels are only available in hard mode.

Salerno Academy
Dark Tower of Xervah
Citadel of Flame
Sunken Shrine
Tombs of Amenti (two Flame Jewels)
Arianrod Labyrinth
Celestial Castle

Artifacts to Keep

Bark of the Dryad: Turns into the Dimension Slip when you obtain your creation gem.
Fairy Bottle: Turns into the Orihalcon with the creation gem. Available in normal mode.
Manuals of Resurrection: Also turns into the Orihalcon with the creation gem. Available only in hard mode.
Eternal Lamp: Avoid freezing.

Unlocking secret items and characters

Play in hard mode, and collect the eight Lotus Gems there. Then play the Seraphic Gate. You have now the access to the locked green doors, which is the location of the secret items and characters.

Avoid environmental damage in Tombs Of Amenti

If timed correctly, you can avoid the horizontal pillars of fire while they are impassable. Jump through the fire where it starts at the edge of a ledge or wall. Note: This does not always work.