In my opinion, the most optimal combination is Medusa - Night Terror - Black Dragon, because Chaos doesn't have a decent ranged unit in the first two tiers. Medusa serves as a strong ranged attacker. If choosing between an Ifrit and a Night Terror, the Night Terror is significantly stronger and more effective in an army. Also, I think the only advantage of Hydras over Dragons is their price. But for me, this doesn't matter because the power ratio between them is about one to three.