If you're having problems with winning an online Player Match and don't have a good friend to let you win, do as I said in the Moving On Op Achievement:
Use Kuma/Panda and stick with his X+Y,X+Y, X+Y combo. It does A LOT of damage and downs the rival character, waiting for more of your combo. It really helped me to get this achievement to do that combo.
Alternatively you could use Kazuya with his Forward+Upward diagonal BBB combo. Like Kuma's, it does a lot of damage and down the rival.
I also found Lars's Forward+Upward A Move, pretty useful, as it can be chained infinitely. You could begin with Forward+Upward B (strong kick attack) and then immediately begin with the Forward+Upward A Move infinitely to max your damage.
Try doing this and, if you have have luck and don't have an Uber-player rival that owns you in no-time, you'll get the achievement at the end of the match.
Good luck everyone :)