In order to do this awfully usefull trick, you a Rock Candy. Once you've found one, buy as many Sugar Packet as you can. Arrange the inventory of the character who has the Rock Candy in way that all the Sugar Packets come first and the the Rock Candy in last position.

Now the fun part, find an easy monster to defeat (be sure you won't kill it one shot!). During the fight, make everyone defend themselves expect those with Rock Candies and Sugar Packets in their inventory. Those with Rock Candies should use them at will! While you use the Rock Candy, only a Sugar Packet will be taken. Doing so, you can boost any of your character to new limits!

Be sure that if you to boost someone a lot, the character shouldn't be about to level up, so you have plenty of occasions to boost him up before he gets a new level!