2 star airstrike fairy, 100 AP (not fully enhanced), using budget units to show that hydras are perfectly killable with average RFHG. SL4 on FN94 and Springfield skill not activated.
Astra's movement at the beginning is important to get the Cerynitis to stop - this allows the RF to retarget to them and get rid of the adds, otherwise they just focus the hydra and your HG get shredded.
(I turned off Springfield skill so that the Hydra would survive long enough to show the proper kiting procedure - if you are forced to use bamboos you can manually activate them for a faster kill when you can lethal the hydras.