[DaDude]

In Sonic Adventure DX , I noticed someone using Knuckles in a video and battled all the Chaos's. I thought he went through the entire story and cut the videos each time at first but then noticed it tallied it up at the end. I knew this didn't happen in story - so there must be a boss select! ... So, wondering, can you battle bosses outside story mode? If so, how?

[yse]

From the start menu, select Trial -> Subgame -> Knuckles -> Boss.

If you don't have it, I'm not aware of what unlocks it.

[Bilan]

Beating Knux's Story?

It'd be pretty redundant if you could fight his Bosses before you actually met them in the story :/

[Marth]

Yeah, that was my video. :D

It's like mike89 said. I never really thought about what unlocks the Boss mode,
but I guess it appears when you finish Knuckles's story or something.
If it doesn't appear then, it will appear when you finish Super Sonic's story.
(but if you haven't gotten far enough to unlock it, you probably wouldn't want to time-attack already)

[DaDude]

LOL, I've never went into sub-games. Thanks. ^_^

Yes, I've beaten SS, etc. So it should be unlocked.