Medals
It is important that you pay attention to your medals and use them accordingly as they are the entire basis of your attack and defense. You have the option to purchase more medals from the shop using Jewels.

Leveling Medals
Level up a medal you like to increase its attack and defense power by spending medals you don't want and if necessary some additional Munny.
Keyblade
It is important that as you complete quests you upgrade your keyblade whenever possible as doing so will boost the power of certain medal types and unlock more slots for medals.

Medal Order
Medals appear in the order they are equipped and continue around in a clockwise order. There are certain medal abilities that will give you a temporary boost instead of dealing damage so if you have a medal that will buff your attacks you may want to consider putting it in the top slot so it is your first attack at the beginning of combat and you benefit from it straight away.
Medal's Special Attack
When you have stored up eneough energy from fighting you can drag medals to the center of the screen to trigger their special attack. The amount of energy you have stored up is displayed in the top-left corner of the screen and to view what the special attack of a medal is just tap and hold it. Before using a medals special attack be mindful of how much energy it takes and how much energy you have remaining.

Magic User Medals
Use the magic user medals such as Cinderella's Fairy Godmother or Fantasia Micky for evolutions.
Non Fighting Medals
Use Huey, Dewey, and Louie to level up the non fighting medals. Although you cannot equip these medals they are still important.
Getting More Medals
As you progress through the game and upgrade your medals you will eventually unlock a fourth and a fifth slot which will mean more turns and more hits.
Elements
Although it will take longer it is still possible to defeat an enemy if you have a weaker element. There are three elements (Power, Speed, and Magic) and each is weak to - and strong against - another. A strong element against a weaker one will do extra damage. More often than not the target in your quest will be the most durable enemy so try to equip at least one or two medals of the element that it is weak against.
Quest Objectives
Try to complete the three objectives you are given before you start a quest as you can get some worthwhile rewards from them.
Loot
There is plenty of loot in the form of treasure chests and items scattered throughout each area when you do a quest. Lots of these items are important for upgrading Keyblades so it is worthwhile spending time exploring and trying to collect them. This is especially true in the later levels where you can find all sorts of offshoots and hidden paths.

Explore
If you keep your distance from an enemy when exploring a level they will not attack. Use this to your advantage in order to collect more loot before engaging your target.
Avatar Board
Don't forget to check the Avatar Board every so often as unlocking more items will grant you permanent character improvements such as health or a larger AP (stamina) bar. You will need Avatar Coins in order to unlock new items.
Avatar Items
It is not necessary to equip the avatar with items you have enhanced to get their benefit. Once you have unlocked something using Avatar Coins it is permanent regardless of what your character wears.