well, i hoarded like 100 adult dragons and counting
Goodness gracious, that's a lot of dragons. I think I misunderstood your question initially; let me try again.
Most of your defense against the Hydra comes from building defensive structures; given how time-intensive most of them are, that's a bit tough. Dragons not assigned to be a soldier don't participate in the raid; they're sheltering and letting the military handle matters. Perhaps that explains it?