Well, you're keyblade has just been destroyed, and you're completely helpless. Stop swearing, and run up to the gates. Darn that Pete! He's locked them! Go back towards where you started, turn left and use the elevator thing. When you reach the sewer waterway, go and touch that blue light. A countdown will begin and you will blow up have to touch the red light. Pretty simple seeming it's right next to you. But they get harder. From now on we know this game as Blue Light, Red Light. Keep travelling through the waterway, playing Blue Light, Red Light, until you reach a large blue diamond. When it lights up, activate it, and the gates will unlock. Go back through the waterway, up the elevator, and through the gates. Unfortunately, Pete is waiting for you. He summons a big cage of Bug Blox and Heartless. Just remain calm, and run around like a maniac hoping you won't die. Eventually, you're friend from the real world Goofy will come and save you. Defeat the Heartless with his help, then go up the stairs, and enter the door with the Heartless Emblem on it. Darn that Pete again! He's locked this door as well. Go back a bit, then head left through a brown door. Touch that shiny thing on the table. Yay! You got an Emblem Piece! Head back through the door and-- wait! Is that Donald?! Head back out and follow him. He's gone to the waterway. Go back to where you unlocked the gates, and you'll discover a big wall of blox. Get Goofy to break them, then touch the Blue Light. You'll play Blue Light, Red Light, and then catch up with Donald. Thanks to Donalds blundering, you can now reach a new emblem piece. Grab it, then head back up to the Library. Examine the spiky thing, and go through the entrance. Keep progressing through the Libraries, playing Blue Light, Red Light on the way, until you reach the Upper Level of the Entrance Hall. Make sure you grab that Rare Prize Block in Library 2, as it's got a pretty powerful Finish Command in it. Play yet another game of Blue Light, Red Light, and grab the third emblem piece. Now jump-yes jump-down to Lower Level, and head out the door. You should see a Blue Light. Set your command to blox, then activate it. Go and grab that Rare Prize Block, and get the last emblem piece. Yay! You've now got all four pieces! Go back to the Emblem Door, and travel to the Great Crest. You'll have fight a hoard of Heartless on your way, but they're no big deal. When you get to the High Tower, you'll get to play our favourite game. Yep, Blue Light, Red Light. Once you've done, walk right up to the pipes, and start looking for the sector. It's right behind those moving Danger Blox. Donald and Goofy will be in this sector with you. Once you've cleared it, go past where the pipes were, and prepare for one heck of a boss battle.
Pete is the easy boss. And now that you've got your keyblade back, you can pulverise him! When Pete rolls a ball at you, make sure to block it, as it will stun him giving you a chance to counterattack. Avoid his exploding rocks, and if he generates a shield, just wait until it wears off. Oh, and if Pete actually attacks you with his fists, block then jump, because his last attack will create a shockwave which could be your downfall. Also, watch out for Danger Blox. They will randomly appear during the battle, and are extremely annoying.
In phase 2, Pete gains his signature move D-Thundaga, where he will transform into a ball of light, and chase you around the area firing lightning bolts at you. Do some mad Dodge Rollingto avoid the attack all together. He will use this attack several times during the battle.
Data Riku is a difficult boss. The most simple strategy is to block, hit, and run. In the First Phase, Riku will go ape on you, trying to land a blow whenever possible. With a well-timed block, you can stun him, then counterattack. With this strategy, you'll easily defeat Riku's first phase.
In phase 2, Riku will teleport directly above you if you're too far away, then slam into the ground sending several Bug Blox after you. If you time your jump well, you can avoid this attack all together, and land with no damage.
In phase 3, Riku gains his most deadliest attack yet. Dark Aura. He will shoot around the area in a shield of darkness. It's impossible to tell where he's going to go, so randomly Dodge Roll around hoping you won't die.