Not required to kill all the ouroboro?

I think the monster just counts as a single being for all 4 heads, or some strange thing. If I remember right, when it fully dies there's a single puff of black smoke in the middle of the area and not one for each head at the places the heads are at from that one rp event against the ouroboros. I think that, since the heads don't normally actually really die until all 4 heads are killed, maybe hydra transmutation messed with that and fully killed it.