To easily kill the politician, use a Mosin-Nagant 1891/30 and enter sniper mode. You will now be using the scope. You will see an island and the politician near his microphone. Simply target him and shoot. After killing the politician, go inside again and follow the stairs. At the last floor you will see a door and dog sitting near it. There are around four to five dogs. Kill them first from inside, then go out. You will reach a huge gate with a big lock which you cannot open. Shoot the lock on the door to exit.
After killing the first four men, turn right, go through doorway, and open the barred door. You will hear someone say "Hey, what's going on?". To your right is an open door with two men in it. One has a S/W 10 and the other with a pump action shotgun. After killing them, look on the floor near the file cabinet. You will find a crowbar, which can be used instead of shooting off the lock at the end. Equip the crowbar and press ACTION and you will see a cinematic of Tommy prying the lock off. There is also a door to a balcony that is very easy to shoot the dogs from.