I assume that you're talking about Demon King of the Six Heavens. If so, then the answer is that the general is pretty useless. A zero star UR will get destroyed by a four star SR. Using UR EVO or Universal EVO cards to get Demon King of the Six Heavens to four stars is a massive waste of very scarce resources that would be better used on just about any other UR general.
It's not a fixed 2.5x damage bonus. It's a small chance at it that relies on having high intelligence, which you won't because the general is zero stars. So the real answer is, it doesn't matter what class you set the general to, as you don't want her on your team.