This mission consists of three phases:
- you have to defend your money safe from Sonny's men
- you have to kill Lance
- you have to kill Sonny
Before you start the mission I suggest you get a Rhino tank and store it in your mansion garage. It will make things much easier.
The first part is easy. Just stand behind your desk in your office and shoot anybody coming in. If nobody comes in you might want to get out for a while to lure them in.
After a while, you get a message to kill Lance the traitor. Go out of your office, and a cut scene will play. After it ends, don't try to kill him there: if you shoot him he will run away, rocket launcher and grenades don't do any damages, and there are too many people shooting at you. Instead, follow him to the corridor and up the stairway to the roof. Before going to the roof you might want to go downstairs first to get body armor, health, and ammo.
On the roof, go straight to the left and get the adrenaline pill. That will help a lot. If you're still healthy enough you could try taking out the men from there, or you could turn right and run straight to the helipad. There's a health icon there. Also from the helipad you could toss grenades and that will take out some of them.
To kill Lance you need three shots, the other men take one shot. After killing Lance, don't go downstairs straight away. Get to the front side of your mansion, look down and you'll see three stretch limos. Destroy them with rocket launcher.
Now get back to the stairway and head downstairs once more to get a body armor. Get to the roof again, and then jump down to the front of your mansion. Enter the mansion from the front door and run to your office. Sonny will come in and another cut scene will play.
Go to the corridor again and down the stairs to get body armor if necessary, then get to the roof to jump down again, but this time try to land in front of your garage. Or if you don't just get up and run as fast as you can to the garage. Get the Rhino, kill all the men outside, then try to climb the front stairs. If you need help, turn the turret backwards and shoot, that will give you some extra push.
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At the top, turn the turret to the front again and shoot everybody in front of you. This will give you protection and fire power. After a while Sonny will be dead and another cutscene will play. Then the credits will roll.