I just got the editor and I was wondering, how do I do a boss bash without ending the level until after the last boss?

Do you mean a boss rush? If that is the case, just put the key location outside of the view.
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Thanks. What about Bowser Jr.?

Do the same thing...?
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Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

Oh, then either make it the last boss or don't use him at all.
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Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

Thanks!

Also, how do you make it so that you can only advance after you beat the boss?

Try using the method described in this post.
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Nothing to say, so jadnjkfmnjamnfjkldnajfnjkanfjdksan jsdnvj m.

Thanks

There is a trick to continue playing after beating Bowser Jr, as demonstrated here by  MeroMero:

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I could not…

Thanks for all the help!