Your Companion, Mog: When you head out into the miasma, Mog will go with you to help carry the chalice. Call Mog close or have him pick up/put down the chalice by pressing the X button. Mog will not take damage, or physically attack your enemies. If he isn't carrying the chalice, he will sometimes stack his spells with your spells or focus attacks.
Painting Mog: If you go visit a Moogle nest, Mog will ask you to paint him. You can paint him blue, green or red, and also trim and wash his fur. Mog's color and appearance will affect combat.
Focus Attacks and Spells: When attacking or casting a spell, hold down A Button until targeting ring appears, then release. You will unleash a focus attack or spell.
Combining Spells: In Multiplayer mode, players can stack and combine spells. To combine spells, players should stack their targeting rings and then cast the spells together. Stacking the same spell makes your magic more powerful, and stacking different spells creates brand new ones.
Enchanting Focus Attacks: In Multiplayer mode, you can combine Focus Attacks with spells to execute magical attacks. One player charges a spell; the other player charges a focus attack. Stack both targeting rings, have the spellcaster release first, and witness the carnage!
Radar: In the one player adventure, you can have a radar screen while you play by hooking up a GBA to Controller Socket 2 on the GameCube. The kind of radar displayed is determined by the color of Mog's fur at the time: No color = Terrain Radar; Red = Monster Radar; Green = Scouter Radar; Blue = Treasure Radar.
Spell Fusion: You can fuse some magicite with weapons to create brand new spells and attacks. On the command list, put the magicite or weapons in adjacent slots. Some combinations require three stones rather than two. If you pick up magicite or "phoenix down" when you have empty command slots, it will automatically be assigned to those slots (for one-player and multiplayer modes).
Elements of the Crystal: Crystals can have fire, water, wind or earth elements. The element in your Chalice's crystal affects the whole caravan. You can change the element at hot spots, found inside dungeons. Without the correct element, you can't pass through certain areas, like Miasma Streams.
Activating Objects with Spells: Sometimes you have to cast a spell on an object to be able to proceed. If you can't get a spell to work, try casting fused spells.