Yoooooooo just power on through you're so close. The first part of Neo Bowser Castle is annoying though because of how many times you have to deal with Kamek.
Still, those last couple boss fights are absolutely WONDERFUL if you're at the right level for them--that is, not too weak that it's impossible, but not too strong that there's no challenge. Watch out for the final boss though, it can be wicked difficult.