This is done on the Subway level. Immediately go to the right side room and grab the GPS. This will give you a map of the subway system and show you where the underboss is. Now, use the markings on the ground and the signs with the subway system map to cut off the underboss. Once with the underboss, pull the emergency break on the train to finish the level. The underboss' starting location and path through the subway system will change each time you play the level. It will always be easier, if he spawns of is moving around the middle red line of the subway.