The Hydra event is currently the end of the demo—there's not much after that in this version, just continuing to play. No more buildings, unfortunately; they're coming though!
For the fight, you need at least 10 adult dragons to start that. Otherwise, I'd personally say you should have at least 15, with soldiers, for the fight itself. Most of your defense is gonna come from the buildings you put up after starting the raid. =)