If you didn't save before entering the door you can now Save right to the left as you enter Destiny Islands. Then head forwards and talk to Selphie. Destroy all the Red Blox in the area for some money and then continue down to beach and talk to Wakka.
Use the auto-jump feature to jump from metal blox to metal blox until you reach the prize blow. Hit it three times to get a HP 2 CHIP. Drop down and talk to Wakka again and then equip it in the Stat Matrix. This unlocks the ability Scan. This simply allows you to see the name and HP of your enemies.
Now grab the three other Prize Bloxs on the island. Then highest one contains a BLANK CHIP. Although the Blank Chips don't provide a specific bonus they are still important as they fill in spaces in the Stat Matrix.
Then use the Metal Blox near the waterfall to climb up on top of the shed and jump over to the next area. You'll see the hooded figure once more and need to chase him into the hidden Backdoor.
Power Armlet (Activates the “Attack Boost” ability, which boosts your Strength when you are down to 25% HP) - 90 SP
Strength 1 -- 70 SP
Magic 1 -- 70 SP
Defense 1 -- 70 SP
Blank Chip -- 30 SP
System Sectors are small arenas that are multiple floors and are filled with enemies. You must kill all the enemies on each floor before going to the next. At the end of the Sector you will get a chance to buy chips and other items with the points you earned in the Sector. Many Sectors have bonuses for completing them quickly and doing other things. Make sure to destroy all the blox and dodge as much as possible as getting hit reduces your SP.
After the Sector is cleared and you've bought all your upgrades you'll be back on the Island and a bridge will appear to take you over to Tidus. You'll have to have a sparing match with Tidus but it's pretty easy. Just keep hitting him and it will be over soon. Grab the FIRE from the prize blox before head back to the main island.
Talk to Selphie again before heading to the waterfall to get a DEFENSE 1 CHIP. If you've gotten everything up to this point you'll need to switch out a blank chip for the defense bonus in your Stat Matrix until you open up more of the matrix. Save the game and head to the water fall for the first boss fight.
In the first stage Darkside will slam its arm down into the ground creating a small shockwave and also releasing several Heartless. Attack the arm to damage Darkside. Randomly Bloxes will spread out that you can destroy while dodging falling magic attacks from Darkside.
The second stage has Darkside again pounding his arms into the ground, but this time you need to destroy the 3 Bloxes. This will stun Darkside and he will drop his head. Attack his head to damage him. Darkside will also release a volley of Blox at you from his chest, dodge to the side and then use Fire to blow some of them away for bonus points.
Darkside is reduced to a single blox. Dodge out of the way of the bloxes that cover Darkside. When they expand away from him, run in and hit him hard. As they start to slow down retreat before they collapse on you. Use Fire to destroy blox for bonus points.