Elves are my favorite faction. I really liked the combination of sworn enemies and dark magic. I tried it on the "Frozen Land" map. There, on the neutral territory, there are neutral peasant units, more than 100,000 of them. I cleared them out with a squad of 150 upgraded archers, 70 druids, and 30 ents. The peasants were sworn enemies. Enchanted Berserk Arrow. With a rain of arrows, you cast this spell on all units, and just wait for them to kill each other. I think this can't be done with any other faction. One drawback is that you can only change the enemy in the castle.