For these two achievements, you need to win a total of 50 multiplayer games. "Mystery Flavor" requires it to be against random opponents, while "Sugar On Top" does not. Therefore, to get them to stack, you should only play against random opponents. When you enter Multiplayer from the Main Menu, click on New Game and then Random. Wins in this mode will contribute to both achievements. Winning 50 games is not that difficult. There are a lot of bad players just trying the game because it is free. Just keep at it. Playing online is also a good way to boost XP, as you get 25 XP just for playing a game, win or lose, whether you even try or not in the game. Additionally, in the screen with New Game at the top, you can scroll down and see every opponent you ever played. If there is a bad opponent, you can click (and hold) on their name and challenge them to rematches over and over. If they accept and lose you will get the victory. Even though they are not technically a random opponent, it will count.