I'm not sure whether or not this is true, but due to the fact that Sora, Donald, and Goofy split up right before Sora fights the 1000 Heartless, Donald and Goofy probably fought 1000 each as well.
Donald And Goofy Seperate from Sora to battle heartless in other area's of hollow bastion. That is only a prediction because there is no explanation
Story-wise: To effectively defeat more Heartless. Game-wise: To challenge Sora to defeat all 1000 Heartless by himself.
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 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.
Showdown at Hollow Bastion
In the battle of 1000 Heartless, the Nobodies easily defeated the Heartless, so Xemnas should probably be stronger then Fake Ansem.
nothing special happens, you have to defeat 1000 heartless in one of the missions in hollow bastion
It's just before 1000 heartless fight. and after Demyx fight. When Goofy gets hit on the head u go automatically here
The only time when Cloud goes in your team is when the 1000 heartless comes and Cloud fights with Leon and so on. You can't get him actually in your team with donal and goofy
It is in hollow bastion when darkness surrounds the world.
After the 1000 Heartless Battle, go to the place after the area where you fought them. Press the reaction command "Approach" on Sephiroth. But beware, you must be at least Level 80 to fight Sephiroth.
After you have completed the 1000 heartless fight,you can retrace your steps later on and one area before the 1000 heartless fight area you will end up in the crystal fissure with about 4 treasure chests and a save point.