To successfully pass the mission, stop the bus by stressing out the perp. To earn a commendation, both user and bus both suffer 50% or less damage and you don't fire a single shot. The only way to get the bus to stop moving is to pull up behind it with your sirens on and wait until the blue "stress" bar fills completely. Keep as close to the bus as possible but try not to damage it. Make sure to avoid hitting the vehicles that the bus tries to force in your path.