Spirits: This varies depending on play style and what part of the game you're at, but usually, Skelterwild, Lord Kyroo, and Frootz Cat are a good team for general use (with Frootz Cat on the bench). Some people like to swap in Yoggy Ram for Lord Kyroo for boss battles, because it uses Spirit Roar more often. I've never needed to use him, though, even on Critical mode. Keep in mind that you can't upgrade their rank once they're made, so try to make the star rank right away (find Risky Winds forecast to use less pieces).
Spells: All the fire spells are good choices (Firaga, Firaga Burst, Dark Firaga). The Balloon spells are also pretty popular, although I don't use them. Make sure to also have several Curagas in your deck.
Abilities: For the end game, you'll eventually want Superglide for Sora, and Dark Roll for Riku, as those are their best ways to dodge attacks (Sora also has his own dodge roll, but it's not very good). Dark Barrier for Riku is also pretty good to get, as it blocks attacks from all directions. The most essential ones for both of them are Second Chance, Once More, and Leaf Bracer. Fire Screens can also be useful, as there are multiple bosses that use fire (Riku will likely face 2 in a row). You'll also need Lightning Screens and some status blocks if you plan on taking on Julius.