Hey Rex,

The way I did it was when you had the cut seen with Sonny don't go into your office, turn around and run into the stairwell again and go down to the bottom level. It is handy to have the mini-gun. Move through the corridor until you can just see Sonny. Then let fly with the minigun as it just disintegrates him in seconds.

It'll then go the cut seen and its all over. At least that's how I did it after a few attempts.

Good Luck