Mission: Keep your friends close:

As soon as you get control of your character stay in your office and protect
the safe with a colt python (handgun that kills people in one shot) then when
it tells you to kill lance just sprint up to him and chase while ignoring
everyone else. continue chasing lance up the stairs and kill everyone in your
way(Just before you go on the roof you may want to use cheats like restore
health and restore armor). When you get to the roof run towards the helicopter
pad and pick off everyone slowly while hiding behind the wall near the
helicopter pad.

When you kill lance run back down the stairs and get the large
wide stairs in the huge hall. Then a cutscene with sonny forelli will appear
and as soon as that is over run away from him and run back upto the rooftop
where you killed lance, then get out your rocket launcher (use cheat if necessary,
and remember to refresh your health and armor). With your rocket launcher stand
on the edge of the rooftop on the tiles until you can see the !
three limosines that spawn enemies then blow those three up to make life easier.
Then jump all the way down to driveway but make sure you re well away from the
front stairs when you make your jump. When you hit the ground remember to kill
anyone attacking you (refresh your health and armor with cheats if necessary).
Then approach the from the large stairs( you should be outside now). Then fire
at Sonny like there's now tommorrow with rockets it took me five rockets for
me to kill him.