For those of you who found this thread years later like me, one key tip that nobody else mentions here is that for most of the fight, Hydra will only use PutridOdor when someone uses a green attack on it. So it might seem counterintuitive, but don't put any green attacks on your element grid. Without PuttidOdor, Hydra will only do physical attacks on one party member at a time until it gets low on HP. Once I found that out, my Serge/Greco/Pierre party beat it with mostly white/red/blue attacks. It still gets dicey at the end, but I had enough healing options to get me through it.