Its actually very easy to fight this boss:

Get one high health stego and one good damage deinonychus.

Have the stego attack the hydra on aggressive.

Since its very fat the hydra will only attack it and not your fluffy raptor.

And then just go behind the hydra inflicting bleed damage on it.

There you go! You defeated the many headed boss.

Will all its heads attack at once? Or will they bite like five Dino’s are attacking you…

I wonder..