Hydra + hardened scales + panharmonicon is a 3 mana 8/9. Play it on turn 5 after panharmonicon, with X=3 and it's a 16/17 trampler.
OTOH, by itself, the doubling side gives you merely a creature that would be an average limited green uncommon beater. Funnily, if you blink it, the hydra comes back as four legs without a head.
Love it.
Or, in standard, Hydra+Nissa, who Shakes the World
Nice with the new Giant Growth