Maybe he's more experienced. Orcs usually deal a good amount of damage at the beginning of the battle, and rings can help a lot. Try to find a weapon with power or at least a pendant of mastery. Level up the Images skill and get to level 3 of the Wrath skill first; it will be very effective. If that doesn't help, then I don't know. Practice your tactics. It's useless against Centaur Knights.
Take a look yourself; with levels 2 and 5 creatures, if the Centaurs have accuracy and you're playing against Garun, then kill a couple of units, and you'll see the result.
Against Paladins, let the Pawns attack first, then you can use Daughters and Executioners. Don't forget that there are other units, otherwise, you'll take out one, and the others will finish you off.