In your opinion, which faction is the strongest in Heroes 5? Let's discuss the advantages and disadvantages of each faction.

Why do Hydras survive? I understand they have a lot of HP and high defense, but against Armageddon?
Also, I think the hero should be leveled up specifically for the final battle; otherwise, you might lose out on rapid development, and then the last battle will be over too soon.

Hydras have 125*6=750 HP per week.
The Black Dragon has 240*2=480, and considering the irresistible magic, which can consume up to 75% of magic resistance... that's how it turns out. Besides, the Black Dragon tries to kill the opponent immediately, especially if the battle is going to be decided by Armageddon, while Hydras... who needs them? They cause the least damage – they are not targeted. So, in practice, that's how it works – the Hydras remain.