Anything lower than expert is unplayable if you ask me, on any instrument (not singing). The most important thing, and I guarantee this will help just about everyone having a problem moving from medium to hard), is to move your whole hand to the red, yellow, blue, and orange buttons, and just slide your index finger to reach the green button. Also, "pinch" the strum bar for playing the faster parts so you can strum up and down. I guarantee you'll get the hang of 5 buttons in no time - the speed is your next challenge, but it's not that big of a deal until you move up to expert.