Food is awarded for completing levels and opening treasure chests. They are usually stat boosters that can increase your character's attacks, help their defenses, and modify other aspects. Consider using food only on characters that will be used for awhile.
Potions
Before starting a battle, enter the "Prepare" and equip any desired potions. Consider equipping a healing item to help with levels that comprise of multiple enemy waves. Using potions on Jake is effective, because his Taunt ability can cause enemy attacks to be directed at him instead of other characters.
Special abilities
Each matched-color triggers an attack for the character currently queued up to attack, however the colors matched also contributes to powering up a character's special ability for the corresponding color. Regular attacks deal little damage. The majority of your damage will originate from special abilities.