Not only is the quantity of your army and spells important but also their designation.
All units are divided into five classes: Warriors, Defenders, Archers, Mages and Support. The pentagram above displays that every class has an advantage over ones and loses out to other. It's essential to form one's army in the way it could stand up to any army or to a definite army. This way you will have a better chance to win any battle.
The same applies to spells. No doubt it's nice to have attacking spells in your armory but if the army lacks a healer (or healing spells) your units may not survive hard times they may encounter at battle.
Some other important battle hacks:
– If you approach an Archer or a Mage face to face it won't be able to damage units at distance, just those close to it. Otherwise it'll have to step aside.
– Sometimes it pays off to delay your turn so the enemy would get closer and you'd be the first to attack. The turn will get back to you before the end of the round.
– You'd rather not gather your troops around some units as they can damage not only their target but also other units nearby at a time.
– Knowledge of your enemy is the halfway to success, so don't forget about recon.