This mission is currently bugged. When you to fight Matt Miller after a few shots the game will glitch and Matt Miller will disappear. Retrying the mission will not fix this.
There's good news and bad news to this
Good news: You can clear your cache and play this mission offline to surpass this mission with no problems
Bad news: When I did this I did NOT get the achievement. Hopefully they'll patch this soon.
EDIT: 1/22/2012 I just did this mission after the Genki Bowl DLC was released online with no problems. It appears to be fixed.