After accepting the contract, the skull marker will appear on the map within a rectangular restricted area zone slightly to the northeast of your current position.
Run to the viewpoint marker to the north and on the eastern side of the church building, hop up onto the wall. Fortunately, this wall runs parallel to the restricted area and from here, if we activate Eagle Vision; you should be able to tag the assassination target without entering the red zone.
When you are good to go, put the guard on the roof to your right to sleep and quickly run across the street and assassinate the guard at ground level. Enter the stalking zone behind him. Wait for the target to walk close to your position, then sprint out and assassinate him to end the mission.