The Eleventh Hour - Co-Op
Bonus: Disarm the bomb in under 20 seconds to unlock the "In the nick of time" achievement
This is fairly straightforward mission and of course, should be done in Easy mode. Kill all the guys near the entrance then run to the platform. Continue killing the enemies as you make your way to the front of the first train. Take note that after crossing the barricades, the timer will start. Rush forward and you should immediately see the timer countdown ahead, inside the second train. Kill enemies along the way and find the bomb. Complete the quick-time event as fast as you can to disarm the bomb.