[Foodstamp]

The strength of his dominion will determine disease rates. So try to avoid provinces with lots of his candles. I played as MA Ctis in an MP game once and came across a province that allowed me to recruit sages. By keeping my dominion low in that province, my sages rarely became diseased.

Liberal use of province targeting spells and any sort of teleport you might have access to would be nice as well. Put temples on your borders.

You're going to suffer attrition but if you are able to strike key provinces without having to march through his territory, that would be preferable. Then as you overtake his dominion, you can mass invade later on depending on his province count and your ability mitigate his dominion.

Remember that cold blooded, lifeless (think golems here) or with swamp survival is immune to miasma. I don't think poison immune units are resistant. Other units to consider might be clockwork horrors, claymen etc depending on how far along everyone is in research. I mention these because you probably have decent access to an earth gem income.