Instructions
Scan the battlefield each turn. Enemy forces will often emerge from the fog of war every turn, so you will need to strategize and adapt your plans to the enemy's new forces.
Move healers with a group of fighters around them. Healers typically cannot move as many spaces as normal units, and they are typically much more fragile than combat units. Keeping them close to units with high strength and defense can protect the healer while also providing the benefit of the ability to heal combat units low on health.
Level up your characters evenly. Experience is given to units anytime they complete an action relevant to their class. For example, a healer gains experience every time they heal or a fighter defeats an enemy unit. Keeping your forces balanced, as opposed to primarily using only a few of your stronger characters, will allow you to combat the dozens of enemies that you will have to fight in later levels.
Purchase plenty of items before you enter each battle. Shops can supply you with potions and other helpful items that can keep your units alive, even when in the brink of death.