An easter egg in the game is, when selecting a stage in story, if you click on "other" you can usually select between "info" and "copy". The latter makes an editable version for custom stages in your folder - including most boss stages. Stages that wouldn't work properly in the editor only show the info on selecting "other" though. Javrax appears in Savant's story.

That should cover MOST of that. The Flying Enemy Desibon that shows up in the hard's "Boss Stage Remix", is an out of editor edit of the Enemy Desibon's type. You can also copy these stages if you want to mess around with the pack's oddities in editor.

I've got a question to ask in the stead of my little sibling:

How on earth did you do the bosses? It sounds like you've used a boss which normally only shows up in the endless mode, and we both how no idea how you did that.