First you need to be on No Mercy, Subway mission, easy, then kill all your partners and run through the mission until you get to the subway part with all the derailed trains.

Then if you have not already met with the tank then keep walking and when uyo find him bring him to that train that is the 2nd back from the door that leads upstairs.

If you get in and stand right up against the door he should walk up on the other side of the door and he shouldn't be able to get through, so just kill him!