This is a walkthrough for the mission Just Business.
(Note: Having Carl's Micro-SMG skill level at 100% (Hitman) level will make this mission slightly easier as Carl will wield two machine guns during the firefight and the chase).
First off, drive to Big Smoke's house in Idlewood. Walk into the red marker, watch the cutscene if you will, after the cutscene your view will change to Big Smoke's Glendale (note: whatever vehicle you arrived in will be lost). Now follow the marker on your minimap to the Atrium in Commerce; it's a short drive, so don't drive onto the red marker immediately if you want to hear all the dialogue in the car. Stop in the red marker and watch the cutscene.
After it you will be inside the Atrium. Smoke will yell at you to get in cover. Now for a gunbattle with the Russian Mafia. A health bar for Smoke appears on screen and you need to take everybody out before Smoke is killed. You are automatically given a Micro-SMG so use it to shoot the Russian taking cover to your left. Then shoot his friend who is taking cover a few meters behind him. Look up at the balcony above the goon you just blasted to see another guy taking cover behind the balcony. Waste him. There should be several goons taking cover behind the Atrium's main fountain so blast them too. After there dead move out of your current cover and take a new cover behind the fountain. Watch out for a Russian who comes up behind you. There should be a Russian coming down from the stairs where Smoke has moved, shoot him. Turn to see Russians taking cover on the other side of the Atrium. Kill the two that are in cover and the other one blasting out into the open. Look up at the balcony near the stairs and shoot the goon up there. After that Smoke will tell you to follow him, but before you do, quickly race around and pick up ammo and cash from the dead bodies (which disappears quickly). Also, duck into the alcove where Smoke is waiting for an armor pick-up, if needed. Then, approach Smoke.
You will appear outside the Atrium. Shoot the nearest goon taking cover near you and the other goon behind him. There will be another goon behind him and another one running up the stairs. Waste them then turn left up the stairs instead of going straight ahead. Once up the stairs waste any goons there. Look to your right and waste some more goons. After they are dead you'll only have a second or two to pick up the nearest dropped cash and ammo (or no time at all if you shot all the goons from a distance).
In a brief cutscene that follows, you and Smoke jump on a BF-400 and drive out of a parking lot, followed by a pair of motorcycles.
Once you are out you will be on the back of the bike so you can shoot any thugs that follow you. You have no control over steering; let Big Smoke handle that. And for most of this chase, you'll need to keep looking behind. Smoke's health car is now replaced by a health bar for the bike; if it reaches zero, the bike explodes and the mission is failed.
There will be a bike behind you, target the riders instead of the bike to take it out quickly (this is easier said than done as auto-aiming is disabled during the chase). After that a Packer truck will come after you. Don't bother shooting it, its indestructible. After Smoke complains about the traffic another bike will come after you. Shoot the riders off. Smoke will do a jump over two buses (the game awards a modest insane stunt bonus for this, even though you aren't driving). After that the pursuing Packer will smash into a bus and stop. Smoke will then halt the bike as you come to a three car roadblock. Change your view to the front and keep shooting the middle car until it explodes and destroys it. Then turn to the left of the bike and shoot the two goons near a car in front of you.
Smoke will then drive the bike down the empty flood canal. Return to your rear view as two more bikes appear. Waste the riders. Another bike will then shoot of a bridge and land next to you, take it out. Then ANOTHER bike will jump off ANOTHER bridge. Take it out too. Then two cars will come up behind it. Aim for the gunners poking their heads out the windows to render them useless, you don't need to continuously shoot it as the driver can't do anything anyway. After that a cutscene will show the Packer jumping off a bridge and into the canal. Again don't bother shooting the Packer, as it is indestructible. Two bikes and car will also appear behind you. Kill the riders and gunner. Two more cars will shoot into the canal, one of them will get blown up from a car dropped by the packer, the other one rams into another car and blows up. Watch a ramp for more bikes. Shoot em' up, and Smoke will drive up one of the canals side ramps. Another bike will jump onto the ramp, pop it. A bike and car will come up behind you. Blast em. Then a cutscene will show Smoke driving tha bike up the Packer's ramp and the cars exploding. There will be two more stationary cars in front of you. They blow up with one shot so shoot them. Smoke will drive up to the "Old Sewer Tunnel" shoot the two thugs in front of it the shoot out its gate and Smoke will drive through. Then four bikes will come up behind you.
By this point, it's likely the bike's "health" will be very low, so you need to treat the final portion of the chase carefully. If you can shoot out of the drivers, do so. Otherwise, there are two sets of explosive barrels that can be detonated, but care must be taken so that your bike isn't caught in the explosion.
Smoke will finally drive into a parking lot near the beach and the mission ends (Carl will need to find his own transportation, but there's usually a car or two parked nearby).
The reward for this mission is increased respect (plus the few dollars awarded by the insane stunt jump)