[VedalkenBear]

Viajero: Waves and waves of undead. MA Ermor should always have lots and lots of undead chaff to throw against anyone, and as you point out, they're all Poison-immune.

The problem comes when those Hydras are backed up by their mages (who are also all Priests). You'll lose your Undead in droves.

So, some suggestions. First, get some Undead Horsemen. (IIRC, Holy-3 priests or higher for MA Ermor can reanimate them.) Set them on the flanks with Attack Rear. Worst case, they tangle with the Hydras, and since they're Size-4, the priest spam on Pythium's part won't hurt as many units. Best case, they may take out the mages. I forget, but if MA Ermor still has the Equites, then you could mix these in with the Longdead Horsemen and make it even better.

Second, research Terror. Spam it on the Hydras. If you're lucky, the mages won't have Poison Immunity, and if you can break the Hydras, you stand a pretty good chance of killing the Mages just from the Hydras' own poison cloud. Another option is Paralyze. Soul Slay and Mind Burn are not that useful, because of the iterative nature of the Hydra. However, Paralyze shuts them down hard. Skellyspam will make it that much harder for the Hydra to clear out all the Undead.

Are these Hydras getting buffed at all? What other units are on Pythium's side?