Medium difficulty is not to difficult. You can't to start also on Hard difficulty is not to difficult...
Chapter 15: Per Aspera Ad Astra
Leo and the Tongs want you dead, but are willing to make a deal: Join with Vinci and kill off Falcone. This will be your last stand, one that will make or break you.
At the first level you'll want to kill off the Falcone mobsters that approach closest. They will drop powerful machine guns and shotguns, which will be very useful for killing off the others. When ready, move along the observatory's outer ring, stopping to take cover and kill any enemies that approach.
Once inside, there will be two major problems: enemies on the roof shooting down, and a gangster with a few grenades. Take out the roof enemies first. The grenade thrower will eventually run out of stock, leaving you free to shoot them with minimal risk. You'll want to move up the stairs to get the next wave's attention, but scurry back to cover, as they mobsters will charge you.
While on the roof, be sure to scout around for some of the final collectables. When ready, enter into the second dome. You'll be descending down a long flight of stairs. One of the enemeis will charge you, so kill him off right away. The display cases in the center don't provide good cover, so stay along the outer walls.
With no form of backup, you'll need to to take along time resting to keep your health up. Never advance ahead too far, as Falcone's men will repeatedly outnumber you, typically sending at least one enemy straight towards you. Your shotgun is the best for fighting these close range enemies, while the machine gun and magnum are great for enemies that stay back.
You'll be aided bt Joe during this final battle. Since the room is a full circle, you need to move to the outter ring to have any hope of finding cover.
The small control booths are a great place to set up camp. You can guard one side, and joe will cover the other, allowing you to easily take out Falcone's guards.
When all of the cronies are dead, Falcone will pop up from the center with a toting machine gun. There is no need to move from your location, just headshot the mob boss to take him down. With your target killed, you'll have completed all of Mafia 2.