Once you are able to upgrade character units in-between missions, be sure to carefully watch how many stats point upgrades your character receives every time you pass the 100-point mark. Since Fire Emblem randomizes the upgrades, make sure you maximize your character stats upgrades by resetting the game and reloading from your last save point if no or few point upgrades are given during level-up. To this end, make sure you save your game after missions and every time one of your characters receives a satisfactory bonus.