i swear honestly it is not that hard!!
just a tip: if you need the achievement where you have to beat the expert on default settings pick jesper! he is the best to use believe me, only hard ones in the beginning were jung soo and mark because i cannot serve the ace consistantly but the other ones you can!! sometimes you have to stand somewhere else than the match before because you play another player..
but send me a msg if you cant beat them i can upload a video on youtube for you when i record??
it might help you!