Let's discuss how to develop a hero instead. I've realized that for a rush strategy, or for defending against a rush, in general, for "Key to Victory," you can get by without Wave of Arcana, Dark Mystic, and Elemental Chain up to level 15. You need to upgrade Unstoppable Magic, Education, Chaos, and probably Sorcery. You need to take +2 to knowledge from Education, and not upgrade Wave of Arcana, Elemental Chain, or Dark Mystic. Ideally, you should aim for Ice Cold to be able to destroy the ballista and try to kill the opponent in an open battle. I think that's the only way to counter a rush.
Who else has ideas on how to kill the engineer?