So, for New Year's Day, Nexon Japan set up an event with an NPC in Mushroom Shrine that allows you to make a donation of 10,000 mesos. For just 10,000 mesos, you get a random medal. I THINK the best one with normal stats you can get is +13 all stats and 3 weapon/magic attack. At least, that's the best one I've gotten.
However... this one exceeded all expectations :
+25 weapon/magic attack at the cost of 99% HP and MP.