A very good skill for a demon is Inner Flame (or something similar, in the fire branch, it adds fire damage to the unit). We put this on the Lords of the Abyss (preferably develop fire magic to the third level - you need a mage hero) and go ahead! It's better to have the rest of the troops take a defensive position (well, except for the Destroyers and archers) and call for reinforcements. If you have a mage hero, you can cast several more defensive spells on the Lords, including healing. It's also good for a warrior hero to take Counterattack III and Reinforcements III, and the latter should also be cast on the Lords (definitely take the fire bonus for the warrior!). Personally, I defeated Necropolis in a duel this way, although it was in the beta.