After defeating the Demyx's water creatures in Olympus Coliseum you go to unlock Meg, who is trapped in the underground because of Hades.
After defeating Demyx in Hollow Bastion, you continue onward towards the Great Maw where you fight the 1,000 Heartless.
If I remember correctly, you head over to Twilight Town then go into the mansion where you access the computer that takes you to the Alternate Twilight Town.
There, you continue on and you'll find a portal that will take you to the last World.
A tye die colored room to fight a big purple heartless called bahemoth. but ive fought him a million times in the Olympus colluseum, so i can honestly say, he's not that hard to beat! :)
You just go on and keep fighting people and getting places and soon it will take you out of it
You have two choices,1. You can go to the fairy godmother because you got a Fireglow and make a new summon called Mushu. 2. You can battle and defeat Dark Riku.
Either Olympus Colliseum, or Deep Jungle.
back to the town square
Showdown at Hollow Bastion
I do not think that you can play as Riku at all, BUT you do get to play as a Heartless in Hollow Bastion.
With your gummi ship.
In Kingdom Hearts 2, the Battle of 1000 Heartless occurs when Sora has to fight 1000 Heartless alone in Hollow Bastion. It's easy enough; just keep using action commands and you should be done easily.
Hollow Bastion/Radient Garden
The thousand heartless and Demyx. Those are the main big battles, you also battle Sephiroth once the name changes to Radiant Gardens.
In order to remember the true name of hollow bstion, you have to destroy the MCP.
No one except Sora Donald and Goofy
In Hollow Bastion/Radient Garden
You have to beat radiant garden/ Hollow bastion foor the second time and also space paranoids for the second time and then go where you fight the 1,000 heartless and keep on going down and he will be there
In Hollow Bastion, Sora fights 1000 Heartless alone (a gauge tells how many he's defeated during the fight), because he gets separated from Donald and Goofy. After killing all 1000, a cutscene plays.
In Kingdom Hearts, Hollow Bastion was an abandoned world taken over by the heartless and Maleficent. You were limited to only being in the castle. In Kingdom Hearts 2, people have gone back to live there and you are now in the town and NOT the castle. That's why it looks different. Also, Maleficent had not yet taken over the world yet in Birth by Sleep so it is still inhabitable and beautiful.