This page of the Mafia Definitive Edition guide contains a walkthrough for the Dock Block side mission available in the Free Ride mode. You will learn how to get a sniper rifle and how to protect a vehicle in the harbor from attacks.
You can learn about this mission by finding Herbert Baskerville's letter in the Free Ride. The letter is on one of the tables at Salieri's bar.
It has information on where the mission will begin. You need to find a telephone booth located in the Works Quarter, near the entrance to Lost Heaven Harbor.
If think that you may not be able to complete this challenge, consider turning the difficulty to easy. This'll make it easier to keep Tommy alive.
Once the mission begins, you have 1 minute to reach the vantage point and climb to its top. There you will find a sniper rifle, your primary weapon in this challenge. Tommy has unlimited ammunition for his rifle, but the weapon has to be reloaded quite often.
Your job is to protect the truck from gangsters. Shoot from behind the cover in the place where you found the sniper. Remember to fire at red barrels – they can cause large explosions. You should also watch out for enemy snipers (examples shown in the picture) and eliminate them first.
After disposing of the first group of gangsters, you must reach the loading zone within the time limit. You have little time, so leave the vantage point as soon as possible and start running towards the marked location.
You don't have to climb a lookout tower here. Just stand behind the planks set up in the area. Eliminate gangsters who show up near the place where you started the mission.
If you kill all your enemies and the mission didn't end, run to the location where they appeared. It is possible that an opponent got stuck or simply isn't trying to approach the protected vehicle.
The reward for completing the mission is a Gold Revolver. It does not deal more damage, it only has a unique appearance.