Regarding Monshiro's "power," let's say that 40% of the gang-affiliated delinquents in Covenant Academy are under his command. For whatever that's worth. 

Regarding his actual power, Asura was just used as an appearance. I'd put him somewhere around Star Platinum or Akira Kongou for his baseline power. Once he hits his fusion and summon though, we're liable to see something at home in a Hideki Kamiya game.