Starting the Battle
Don't wait around at the beginning of the game. Concentrate on creating a good amount of units at first. Then start taking over various cities while holding the lands that are already conquered. Stronger units can be developed from the outlying cities.
Combat Tips
Don't use the Auto-combat feature. Auto-combat does work, but it is not as efficient as player-controlled units. The player is always smarter then the AI routine. When considering the strength of an unknown city, send in a single unit to scout the area. The unit might be lost, but it'll show the strengths of the enemy.
Economy Tips
Always pick up the treasure boxes on the maps. This will lead to more gold and better items, including those of a magical nature. Watch the incoming versus outgoing gold. If there is not enough gold to fund the empire, units that are desperately needed will not get built.