Suggested setup:
1. Always deploy a healer in the beginning of a battle as they will be very useful against armored units. Save money to upgrade healer to its ultimate form before deploying other type of unit for it will acquire a lifestealing aura once it is in ultimate form which is very useful for himself and other units. And it will also cast heal every so often so when approaching a boss, it can survive its barrage.
2. When healer is in ultimate form (elite healer), start deploying a recruit pikeman and slowly upgrade it into elite lancer(ultimate form) as it will be your main tank.
3. After gaining an elite lancer, hire a recruit and upgrade it into elite berserker(ultimate form) because having a high attack speed reaps the benefit of the lifesteal aura from the elite healer.
4. As for your last tank, hire an elite pikeman(the counterpart of elite lancer) for it has critical strike.
5. Deploy an archer and upgrade it into elite archer(ultimate form) again for its critical strike.
6. Next, deploy a rookie magician and upgrade it to Lord of Ice(ultimate form) for it gives off a protection aura which further increases the endurance of your tanks.
7. Again, deploy a rookie magician and upgrade it into Fire Lord(ultimate form) to increase the attack power of all your units.
8. The formation would be your 3 tanks on front, 3 magicians on the back of the tanks and the archer at the back of the 3 magicians. Make sure to compress them to gain the effects of the auras. Remember: "Strength in numbers"

LAST NOTE: When allocating skill points to the Hero, max out the attack power status first for it will be a tremendous help. (Believe me, it's true)