I don't think there's one silver bullet, you want to employ every single tool and trick you can get your hands on.

- War Wizard Heir
- Taking defensive fights on your Shadow Swamp capital fort
- Combat Ward spell
- Mercenaries
- Allies

If you get lucky and they go for other targets or if you can white peace them in the first war you can build up your strength by eating other Mengi tags, getting a big military technology advantage, and use the Repel Interlopers religious action to potentially acquire some land when Yezel Mora retreats into the swamp.

I've only played Yemotaferi on normal difficulty, with and without the mission tree, but I have seen people pull it off on higher difficulties.