How can I beat the Ermor?

[Archonsod]

Agrajag said:
Which is again an issue if he has PD...

You'll eventually fight his main commanders though, once they're gone the PD commanders are fairly easy to deal with.

Also, some of Ermor's commanders can be quite tough.

Never said it would be easy

Doesn't have to be one on one though, simply having a good combat character who can somehow engage the Ermor commanders at the start of battle can also work. A good bow in the hands of a high precision commander or someone equipped with boots of flying and some of the anti-undead weapons should do it (especially against the AI). Back them up with some bowmen to help finish off the rest of the army as it crumbles and you could probably start taking provinces without taking any casualties.