Alright I need to understand exactly why this guy who was an amazing soldier and incredible enough military mind to take over the kingdom, thought he'd lose against the Dorthraki?
The Dorthraki are terrifying, yes. Especially against levies and other more easily routed and less armored forces. But they are entirely unarmored cavalry. There are ways to deal with that, even in an open field!
He had years to prepare, had the home turf advantage, had tons of money, had access to magic within his kingdom and magic items like that green fire stuff being made in his keep.
In reality, a bunch of pikes, archers and fighting on rocky terrain would remove most of the dorthraki advantage. Could have dug small trenches too, that would have broke a calvary charge.