You're going to want to synergize you abilities and keyblades if you want to get the most out of them. Generally, you always want to have at least one Finishing Plus equipped, Slide Dash, Horizontal Slash, Aerial Finish, Aerial Dive, Aerial Spiral, Flash Step, Air Combo Boost, Combo Boost, and Explosion, as well as all the useful passive abilities (Reaction Boost, Form Boost, Fire/Blizzard/Thunder Boost, Combination Boost, etc).

Never equip both Combo Plus and Negative Combo, it's a waste of AP. The purpose of Combo Plus/Air Combo Plus is to increase the damage of your finishers via Combo Boost/Air Combo Boost respectfully. The purpose of Negative combo is help spam finishers (mostly useful with magic). Only equip Guard Break and Explosion if you have a Finishing Plus equipped, otherwise stick with just Explosion.

Defensively, you always want Guard, Counterguard, the Growth abilities, Second Chance, Once More, Aerial Recovery, and Leaf Bracer.

You usually don't want Berserk Charge equipped, but it can sometimes be useful or VERY broken. Don't equip Dodge Slash, it's worse than regular combo hits.

Air Combo Plus x1-2+Air Combo Boost+Horizontal Slash+Hero's Crest-Strongest air combos in the game. Horizontal Slash is better than normal aerial hits, switch back to the finisher at the end. Air combos are usually stronger than ground ones, while ground combos are faster, safer, and use magic more easily. If you plan on using physical attacks with Valor/Master/Final, this is the setup you want to use, with either Star Seeker or the Rose as your second keyblade. Always equip Finishing Plus if you have it.

Combo Plus x1-2+Combo Boost+Decisive Pumpkin-As above, but for ground combos. This is generally your best setup with one Finishing Plus.

Negative Combo+Fenrir/Rumbling Rose-Lets you rapidly spam finishers. Stunlocks normal enemies extremely well, but you're usually better off with one of the above setups for bosses.

Fire/Blizzard/Thunder Boost+Bond to Flame/Mysterious Abyss/Photon Debugger-Maximizes the damage of Fire/Blizzard/Thunder. Final Firaga is super broken, and boosted Blizzard/Thunder work very well with Stitch.

Fatal Crest/Berserk Charge+Horizontal Slash-You use a Limit/Cure, jump in the air, press X so an attack connects, then mash square like a madman. Can deal stupid damage, never experimented with it much myself.

Lucky Lucky (for everyone in party)-For item farming, worthless for everything else.

As far as allies go, priority is abilities that keep them alive (Second Chance, Once More) and Limits. Goofy Tornado is awesome, turn off all his other attack commands so he spams it.