Keep Your Friends Close (how to beat it without cheating):

Although using cheats, it's relatively easy to beat any mission, it's
a fun challenge to go through the game without cheating (or at least
not saving when you do)
For the final mission, the best gun to use (in my opinion) is the
Minigun, which you can buy from Phil's place for the giveaway price
of $10,000 (but by this point you should have enough money for this
to be no problem). Also buy extra ammo...

When the mission starts, stand at the top of the stais and shoot
people before they even get in the front door... you shouldn't even
get shot once. When Lance comes in, you might have to move back into
the office, because more men will come from the sides. With the minigun,
you dont need to chase Lance at all, just shoot at him (and hit) for like
a second, and he'll die. Same for when Sonny comes in... I beat the
mission like that only losing about 70 armour. It was easy.