I'm having trouble with the bosses at the end of the Shovel Knight any% run (Black Knight, boss rush, the final bosses). I got the magic armour, so I take more damage, which is probably the reason because I could do them a lot easier when I played casually. Is there something that would make the bosses easier for me (like more health or defence or other things like that) or do I just need to get good?

Boss rush overall takes a good amount of time practicing to get down, moreso than any other part of the run. If you grind it out for a while and still struggle with it too much then getting some extra health or going back and removing Conjurer's Coat before the stage can make it easier. I'd definitely recommend putting a lot of practice into it either way.

Also, you should join the discord! We're all happy to answer these kinds of questions there, and you'll get responses much faster. Here's the link: https://discord.gg/0SqABNqwFExE98xR