[Ironhawk]

Speaking from extended use of Hydras, the most dangerous thing for them, believe it or not, is Slingers. I know that sounds silly but its the truth. The trick there is that you can mass SO many of them and they just slaughter all the hydras heads without getting poisoned.

After that, you just need one block of decently strong infantry since the hydra's defense/prot is quite low. They will make relatively short work of them with slingers/archers to back them up. Tho you will likely lose all those infantry, btw.

NOTE: Do not use crossbows. You dont need damage power, just damage rate.