Playing a mage, i used giants, green dragons, black dragons, shaman, and royal thorns. Nuked the main army especially the enemy necros, moved dragons to their spots. The royal thorns were used to spawn more thorns which ate up the corpses. And the archers took out the obelisk in the end of the battle when there were no more enemy troops
Hardly took any losses at all. The royal thorns were the key for me.