You could try making peace with them, but on VH that is basically impossible. But what else would you be able to do other than "blitz" (or attack) them and take the cities?
They want to kill you, they have a place where they can recruit people, so until you take that place, they will continue recruiting people to kill you. If you are familiar with the American Civil War, this is pretty much what happened, the North didn't stop recruiting people until they won.
I would tone down the difficulty personally, medium works really well for me. The diplomacy kind of works, and I replicated the AI bonus monetary bonus that they get in VH (10k per turn) with the EB script.
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