ok here is another tip. play the following BPM's maps, 110,120,130,140,150,160,170,180 etc etc and FOCUS and remember a good pattern for pressing z/x. chances are you are doing it too fast sometimes(often with BPM's below 150).
you can't just simply press z/x and win, you need to listen to the beats in the BGM. really it will come naturally to you, and you will sync better with the BGM beats. eventually you will be able to hear the BGM and your z/x pressing ^^
i can't give you a good help for stamina except keep playing maps with 200BPM and give it your best.