For leveling certain quests are best for getting unit exp. My favorites are 2-7 and 43-6. The key is to always deploy the max number of allies you can because allies actually increase your earned exp without dividing it. Quest exp is only divided between your actual units. The idea with these 2 quests is to deploy the max number of allies to maximize your bonus exp then deploy only 1 of your own units so it gets all of the exp to itself to level up quicker.