If you don’t want to cheese it with a deinonychus, you can actually just shoot and destroy all its heads to bypass its large health pool. When you destroy the last head it doesn’t matter how much health it has left it will instantly die. On Alpha it still had 750k health left when I destroyed the last head. Use high level Stegos to tank damage. I used an ascendant Pump Shotgun with lots of Ammo.

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.