You don't have to give up on objects that have passed you - you can always nip back if you're quick. Use rocks for cover when enemies charge. You won't really need to take any offensive action when it comes to boss fights. Your main goal is to simply keep running and avoid any obstacles that crop up, which in turn will harm the bosses until they're done for. These stages can move pretty quickly, so don't be too bummed if you have to start over again.