Ideally, also – tactics and air units. Then, with haste, the entire army tears the enemy's throat in the very first turn. Almost all of the Fortress's troops retreat when the remaining half of the enemy warriors get a chance to strike back.
If the enemy closes in first, use the Shield spell.
Because taking only 10% damage is overpowered.
Besides, it has long been proven that defense in Heroes of Might and Magic III is much more effective than attack.
That's all. The Swamp faction is awesome. Simple and effective.