The result of a battle is affected by the type of troops, active boosts, the Hero's and Shaman's bonuses, Skills, Reputes, Town Skins, Knowledge, and so on. Maybe your opponent had Knowledge and Skills that had been upgraded more than yours or was using a powerful boost or equipment with bonuses against your troops.

Please note: at the time of the battle, some of the bonuses may have expired, for example, the bonuses from a boost or a Repute. Alternatively, the wrong piece of equipment may have been equipped on your Hero or a Shaman accidentally.