You need to kill a close friend of Martin that currently incarcerated and then escape the police station.
Walkthrough of the Sixth Martin’s Dispatch Mission
In this mission, a number of targets vary between 2 and 4, relative to a number of players participating. The targets are corrupt police officers were affiliated with Martin. They are transported in a convoy of armored vans guarded by the police. You need to eliminate them before they reach the destination point. The distance from each van to the place is indicated by the progress bars in the lower right corner of the screen.
Go to the marked points, and you’ll meet the convoy near the Maze Bank Arena. You need to blow up the vans or to start firing at them and to shoot each of the targets through the open rear doors.
As soon as all the targets are assassinated, lose the cops, and the mission will be completed.
Video walkthrough: youtu.be/P-QmzqScfBk.
The mission accomplished on hard difficulty by two players. Thanks to Eugene for help.