I made a guide in the Guide for Exp leveling topic on how to get medals in a somewhat timely manner as it otherwise takes quite a bit of time. I'll copy it over.
So my strategy for medals in Mirage Arena: Play secret board with 3 players. This keeps the CPU's occupied with eachother some of the time. Buy a panel whenever you get the chance. DO NOT buy panels from your opponents! If given the chance, always take the keyblade rider square. If you happen to get Captain Dark, use Three Dice hands to try and pass a player, passing him on to them. If you get or another player gets Captain Justice, stay away from them or try to get him. If you get the keyblade rider early in the lap, fly to the yellow checkpoint as it is the farther away than the other panels and it makes it easier to get the rest along the way. Level up panels you think an opponent might land on in the near future. Try to take paths where you will get the prize cube prize. Buying Bonus panels is better than buying regular panels because it lets you always have a hand to use and, well, free commands. Even on the lowest GP objective you get quite a few medals from this. Hope this works out for you guys!