There is no single answer. That's what makes heroes interesting – each battle can be played out in dozens of different ways. But there is a general approach (depending on the situation). Here's roughly how I do it:
1. If you have more archers than the enemy, use slowing and forgetfulness spells (let their archers run around too). Move your infantry slightly forward and just shoot them down.
2. If the opponent is stronger in advance, use berserk (it works well in combination with spells that remove morale) and cast it every round.
3. If you mainly have infantry, use haste/blessing/prayer – the main thing is to reach them.