This part of the guide covers Kingdom Hearts Final Mix from beginning to end on the difficulty, Final Mix Proud. If you are playing on a different level of difficulty then these tactics still apply, the fights just might be a little easier for you.
Choose One, Sacrifice Another
Sora starts out in a "Dive to the Heart," a place inside of himself that acts like a dream. You gain control at the Snow White Platform where you must choose one item and sacrifice another. These choices serve as a starting point for your stats and also determine your skill order while leveling.
The power of the mystic.
A staff of wonder and ruin.
The power of the guardian.
Kindness to aid friends.
A shield to repel all.
The power of the warrior.
A sword of terrible destruction.
The choice you really need to make is going to be whether you want +2 Item Slots or want 3 MP instead of 2. Taking the Dream Rod will always net 3 MP at the start of the game and 10 MP at the end. Taking the Sword or Shield will have you start with 2 MP and finish the game with 8 MP. The Shield always gives the +2 Item Slots so choosing Sword has no statistical benefit since you can increase your Strength, Defense, and AP through items to reach their maximum.
The Power to Fight
After choosing your path you will fall to the Cinderella Platform where a Shadow will appear.
Defeat the Shadow and you are forced to fight a few more. With the foes defeated you are taken to another platform. Examine the door then open the chest that appears then push the box around and smash it to receive your first item in the game, a Potion . Break the barrel that appears and you can open the door to reach a new area.
When you arrive on the ship at Destiny Islands there will be three members of the Final Fantasy crew to greet you. They will each ask a question that affects the pace at which you will level throughout the course of the game. Choosing the first set of answers will start you at Dawn, choosing the middle set starts you at Midday, and the third set of answers starts you in the Dead of Night. There are 25 possible combinations based on your choices but you can always back out if you don't get the desired result.
Tidus - "What are you so afraid of?"
Wakka - "What do you want outta life?"
To see rare sights.
To broaden my horizons.
To be strong.
Selphie - "What's most important to you?"
Being number one.
My prize possessions.
Note : Choosing Dusk requires more experience to level than Midday and Dawn until about level 40. Afterwards, it takes less experience to level. It really depends on how fast you wish to progress at the start and end of the game.
Defeat the Darkness
You return on the Aurora Platform. You'll have to fight more Shadows but you are given an opportunity to save. A pathway appears that leads to the Belle Platform where a large heartless appear before Sora.
BOSS - Darkside
Darkside only has two attacks and being the first boss isn't that hard. He always leads with a Shadow Summon attack where he slams his fist into the ground and emits an aura that summons Shadows. Focus on attacking his fist to kill the Shadows simultaneously or you can climb his arm and strike the head. Finishing a combo attack on Darkside's head results in 2 Tech Points so you can gain some extra experience while fighting him. After some time Darkside will use Darkness Orb, an attack that launches orbs of darkness that seek to your location. You can parry them backwards for Tech Points and some damage, but it's best to outrun them. You can lose this battle but the story progresses either way.