This page contains spoilers for finishing missions.
Full armor/health would be nice, but a health pack is easily available if you need it. Bring plenty of ammo.
When the action finally starts, you are in an alley. The guy you are protecting stays hidden and is not a factor.
Your most immediate threat is in the alley across the street, so take them out first. Look for red dots with arrows on them across the street from you and take them out next. Then take your time and clean out the rest. The last guy, on the right, stays pretty well behind cover. A grenade will get him, at which point his reinforcements arrive, but should be no problem.
You may get a red dot hiding from you. One place is in a nook around the corner to the right. Another place is in an alley to the right of the first alley Bad Guys ("BGs") were shooting from.
As you look out the end of the alley, two BGs are on the roof above you and to your left. Stay against the building on your right when you go after any ground-level stragglers. If you have cross the street, run. When all the street-level BGs are taken care of, get those last couple of guys on the roof.
You may have to go back down the alley to get the guy you are protecting. There is a health pack there if you need it. Get into the front car by the alley and wait for four cars of BGs to arrive. If they don't come right soon, try pulling straight out of the street and when you see their red dots on your HUD, back straight to where you started.
If they stop at the end of the street and fire at you, you can stay in the car and shoot back, but after they all arrive, you'll want to crash through them, turn right, and gun it. There is a randomness to the game, but this has been tried several times and always worked.
If you try to run for it immediately instead of using the above approach, they will be on your tail and it will be more difficult to shake them.
Many of the "easy" approaches like the one above do not work in the latest updates. Now when you wait for the BGs in cars, they do not all show up at once and some of them will come pull you out of the car if you wait, so here is another easy method:
Get in the car and bring up the map. A yellow route shows where you need to go, but when the BGs in cars show up, the destination point disappears until you lose them or kill them, so you have nothing to guide you. The route does not take you exactly where you need to go anyway. It has you turn right, go 3 blocks (see *Note) and turn left, which is okay, but only go a block-and-a-half and then look for an entrance on your right with a bar across it. (When looking at the map, you may want to put a waypoint there.) Crash through that bar and go half way across the drive.
This is City Hall, where you are supposed to take the politician when the yellow destination marker comes back, so all you have to do is get rid of the 4 cars of BGs. Get out of the car and bring up your RPG. The BGs will drive back and forth for a while, but end up crashing into the gate at one end or the other of the drive you are on; they will not come down the drive. Just hit them with RPGs. End of mission.
(*Note: when you first turn right, a BG car will come at you from the other direction on the next block. When he gets close, swerve left around him, after which it's easy going the rest of the way.)
If the game gets changed again to take away this easy method, the next easiest is to bring up the map and put a waypoint on the southern-most bridge to the east. At this time, the BGs don't seem to want to get on the bridge. Once they give up, you can take your guy back to City Hall.
You will need full armor/health a good supply of ammo. This mission is not time sensitive until near the end when you open the kitchen doors, which is when the boss tries to flee. Up until that point, take your time and take cover when you can.
When you first get to the restaurant, drive your vehicle down the driveway to the right of the restaurant's awning, go around back and park it tight against the front of the car that is already back there. More about that later. You may notice a ladder going up the back of the building, but don't waste time with it; the door at the top won't open to the inside.
Now walk back around front and enter the restaurant. After the cut scene, go around to the left of the guy and shoot him, but not until the cut scene is done and the manager's blue dot shows up your HUD.
The reason for going around to the left is that another Bad Guy ("BG") will appear in the room to your right and start shooting you as soon as you shoot the first BG if you do NOT move. Now you can move around the wall on your left, start aiming towards the BG in the next room as you move, and shoot him through the decorative molding.
Look through the door beyond where he was and some BGs will be upstairs. Kill 'em. One may run downstairs and get behind the screens on the right. Shoot at the screens to knock them down then shoot him.
It is possible to walk around the bottom floor throwing grenades to all parts of the top floor, but it seems you still have to deal with the same number of BGs. So just run up to the first platform on the stairs and quickly turn and deal with the BGs who have open shots at you. Stopping at the first platform cuts the room in half so that you are not getting hit from all sides.
After you get all those you can, run up the rest of the stairs and turn and shoot the others. After you think you have finished them, cross to the opposite corner of the room, watching out for any straggler BGs, and go through the red office door.
The manager is in the office, alone and unarmed. Pull LT to target him and he will tell you where the boss is, then shoot him. Armor is on his desk, but unless you are low on health, don't pick it up yet because four more BGs are waiting for you to open the door -- three to the far right and one against the left wall.
Instead of running into the room, stand back from the door and to the left of it aiming towards the right and shoot the door open. Push in the right joystick for your automatic weapon to zoom and you can see two of the BGs just standing there. Take them out quickly then move to get the guy on the left wall (straight out the door). The last guy will probably take cover, but if you zoom and shoot, he is easy to finish off.
Now you may as well go back and pick up the armor from the office desk, exit the office and go to the right towards the kitchen doors in the opposite corner of the room. Keep aiming at the doors. If two BGs come out, you can take them down quickly. If they don't come out, shoot open the doors to get things going. If they prove hard to get, grenade them. Your objective is to shoot them as soon as possible and immediately run through the kitchen to the door on the right to a fire escape.
Chances are that the boss has not gotten into his car yet (the one you blocked in) and you can shoot him before he does. Otherwise, Scoot down the ladder, run over to the car and shoot him. If you have trouble getting to him before he takes off, just jump off the fire escape. You may take a little health hit, but the mission is over as soon as you shoot him (but if the BG with him is still alive, he will shoot at you).
If you are not in time and he manages to push your car out of the way (next time, use a bigger vehicle, like an ambulance) and drives off, go down and get in the car you parked there, not the motorcycle that you would have had to take otherwise, and pursue him. This is an odd chase in that the BG's car goes really slow, and you could go out for lunch and he would still be waiting for you. Keep shooting at his car (which has a guy shooting back at you) and eventually it will either burst into flames or you will kill the boss.
Despite all the shooting and BGs, this is a pretty easy mission. The most interesting aspect of this mission is the ability to position a car before the action actually starts and still have it there later. This may be the only mission in which you can do so.
Liquidize The Assets
You will need full armor/health, ammo, RPGs, and grenades. Something that will make the mission a lot easier is an armored car. If you looked ahead in the past, hijacked one and kept it in one of your safehouse parking spaces for just such a mission, then now is the time to go get it. If you don't already have one, you can usually find some on the road in front of Brucie's, though in later updates of GTA4, they do not seem to be as readily available there.
After talking to Gambettis, drive your armored car over to the destination. When you get there, you will see a tall wire fence with a concrete driveway sloping up from it to where you can see BGs milling around. Drive through the gates and try to smash as many BGs as you can. Of course, the BGs will be firing at you like crazy, which is why an armored car is nice to have. Even so, you cannot spend too much time doing this because the real risk is that one of the guys will open the door and pull you out.
So eventually you will have to park it and get out. The place to park it is blocking the top of the concrete ramp with the driver's door away from the action. Then you can get out and have pretty good protection with it from the snipers on the roof as well as the guys left on the the ground.
Your immediate concern should be with any guys on your left. After taking them out, concentrate on the guys on the roofs. From behind the armored car they should not be too big a problem, and then anyone on the ground level will be even easier.
The red dots left after all the carnage appear to be for the five vans you need to destroy, but there are not necessarily any BGs left. You could use grenades and/or RPGs to destroy the vans, but if you don't want to waste the money, just keep firing into the radiators until the vans catch fire and blow.
The reason for the armored car, aside from squashing BGs and providing cover, is to block the driveway, which is the only exit, in case someone tries to get a van out. If one gets away, you fail the mission.
If you can't get an armored car and use something like an ambulance instead, the risk is that the shooting will set it on fire and it will explode and kill you or, at best, blow itself out of the way to clear the exit again. So if you cannot get an armored car, you may be better off using the walk-in approach:
It used to be possible to drive around the corner by some phone booths and smash the fence with your car to be able to sneak into the compound from the side, but this no longer works in the latest updates, which leaves only one approach:
After the cut scene at the compound, smash the fence at the bottom of the ramp, but don't drive up the ramp. Nobody seems to notice, so you can get out and walk through the fence until you get far enough to put your sniper scope on someone. You only get one free shot before the BGs all start firing at you, so the best guy to get is one on the roof ahead. Then take cover, switch to an automatic weapon and grenades and prepare for a long battle.
The real problem with this approach is that if one of the vans which you cannot see down the driveway to the back right decides to make a break for it, you may not be able to stop them as they blast down the ramp. This is where the armored car blocking the ramp would come in handy.
If you see a red dot moving in your direction from the back right, you can target where they will come out and hit them immediately with an RPG, but if you miss or are too slow, you can't RPG them once they get too close to you and they will take off and you will have to run to your vehicle and chase them down, firing enough rounds into their van to cause it to blow up, and that requires a LOT of rounds. Meanwhile, the cops will be all over you.
Bottom line is that you REALLY want to RPG that van before it gets out.