Note: If you fairly divided the 9 districts to your underbosses, all of them will be alive at this point. This will unlock a corresponding trophy and ensuring that the city's power balance is maintained.
Head to Donovan’s motel to trigger the next story mission. The bridge leading to the large casino complex to the north of the city should be completed by now. Now make your preparation and head to the casino for the final showdown. Make your final preparations and make sure that you’re well stocked and fully ready for war. You have to find and kill Giorgi first before finally taking down Sal. Giorgi and what remains of Marcano's men will be Sal's last line of defense against you.
After reaching the casino grounds, plan your approach. You can use stealth kills using your silenced weapons to decimate the guards’ numbers and just prompting them for a search just in case the bodies are found. This will make it easier for you to ambush unsuspecting enemies. Going loud is also a viable and more exciting option but take note that this is just the first batch of enemies you’ll have to deal with.
Use your intel view to keep track of the enemy positions. Don’t stay in one cover or position. Move often and preferably while crouched so you can conceal your position while moving. There’s a scaffolding that you can use to reach the second floor of the lobby. Use that to have the elevation advantage.
After killing the enemies in the lobby, loot the bodies for ammo, explosives, and cash. Make sure to change weapons if necessary. The objective marker will point you to the door leading to the ground floor offices. Don’t proceed there immediately; move from cover to cover since a group of enemies will spawn there. Take them out carefully and precisely then head to the office. There’s a medicine cabinet there and a Vargas painting.
Leave the office and follow the straightforward path until you reach the circular casino area. There are unsuspecting guards here; it’s up to you if you’ll announce your presence or take them out stealthily. Giorgi is found up the office overlooking the slot machines and he’ll take potshots at you using his rifle. Use the sturdy walls and columns of the corridor as cover and concentrate on taking out enemies near you. Lob grenades at those enemies behind cover or close together. Be careful when moving along the corridors since reinforcements will arrive from the side doors. Use blindfire as neccessary.
After clearing out the group of enemies in and around the gambling area, carefully move towards the bar/ stage area. Several reinforcements will appear from the backstage. If you’re behind cover, lob a molotov or grenade to eliminate most of them at once. Loot the medicine cabinet near the stage then enter the backstage.
Carefully move on the stairs and take out the last of Giorgi’s men. He’ll be waiting for you inside the office so throw explosives inside or shower him with bullets until he’s down. Once clear, grab the second Vargas painting in this location, on the couch beside the lamp. Finally, interact with him to exact your long-awaited revenge. A cutscene will follow.