Manticore is a specialist in scorpions; he needs to defeat 1000 manticores to become available. I suspect he is the toughest of all the green and brown factions. By the way, I managed to defeat the first brown faction with 600 pegasi and other units with almost no losses while defending the castle (only a few vipers and bulls were lost). This hero only has earthquake as a spell and lacks offensive/defensive skills, so he's only good for traveling. However, the green factions with meteor showers are a bit more challenging, but they can still be defeated without major problems.