This is probably one of the harder achievements that you will have to go for, and it's only worth 30G. You have to complete every multiplayer gametype, and every race and GTA race. It is best to boost with at least two other friends, and set your preferences to the same (go to multiplayer in the phone menu, and it is near the bottom).
For Deathmatches and Team Deathmatches, you just need to get an assault rifle, and kill as many people as possible. When you're in a firefight with someone, keep moving, make use of cover and roll occasionally.
Mafiya Work and Team Mafiya Work are when you do missions for Petrovic, and you get varying amounts of money for them. It's always best to go for markers close to you, but far away from other players to give you the best chance of completing it.
Car Jack City and Team Car Jack City are when you have to get a variety of cars for Petrovic. When transporting them, try not to damage the cars too much, as you will get less money. However, if they contain drugs, you will get a bonus, regardless of the damage to the car.
Turf War is where the teams have to gain areas of Liberty City, where you get $50 for every one your team holds, every 30 seconds. You want to kill enemy players if they are attacking your bases, but you do not get any money for it.
For Road Races, I've found that having Trucks, more specifically, the Fire Truck will help massively, as I have found out that when you fire water out of the hose (LB), you appear to go slightly faster than the other players due to a lack of friction (or a glitch). This means you get them done a lot quicker.
For Boat Races, the Jetmax and Tropic are the fastest boats, and have the same stats for everything, but I personally prefer the Jetmax. To win, you want to press LT almost constantly, and hold RB whilst taking corners. You also want to avoid crashes with other racers, and don't miss checkpoints as it takes a long time to get back to one.
In Helicopter Races, the Annihilator is the fastest. You want to keep on roughly the same level as the next checkpoint you're aiming for, not too high, not too low, and press the analogue stick forward to get to top speed. Also, don't crash into any other racers, as this will cause you to spin out of control, and you may lose your rotors. The same applies to hitting buildings.
The Free Races are difficult, but possible. When you start, while it is counting down to the start, set a waypoint on the finish as quickly as possible. If you start in the middle of a road, always check behind you, as cars occasionally spawn there, and you can get a headstart on other racers.
Cannonball Runs are the hardest race mode to complete. They involve getting to four other checkpoints, and are difficult to complete without boosting. Again, check behind for cars spawning, as you can get a headstart. Always set a waypoint on the next checkpoint as you drive through one, so that you don't get lost in the city.
GTA Races are harder, as you are constantly being shot by other players, especially at the beginning, as there is usually a weapon spawn near the starting point. You also want to watch out for people who set up near corners and will just massacre you when you turn. Or people who just decide to drive the wrong way around the track
Cops 'n' Crooks has two modes, and for each one, you have to win as Cops and Crooks for them to count. Neither mode is particularly difficult, but they rely on having decent teammates.
All for One is where the Crooks get a Boss to safety, a boat or helicoptor, while the Cops try to kill them. If you are the Boss, stay with the majority, and if not, then stick on the Boss at all costs. Sacrifice yourself to save him if need be. As the Cops, just head straight for the Boss and concentrate only on killing him.
One for All is where as many of the Crooks have to get away from the Cops, and there is no respawning. I'd suggest bailing and making your own way there, just so the cops are chasing multiple vehicles. As the Cops, bail and get your own vehicle if the Crooks have split up, and go for the guy the other Cops are ignoring.
Deal Breaker is where you have to get to an industrial plant in the south of Alderney, from a house in the north. There is a helicopter spawn in the centre house on Owl Creek Ave, however, it does not always spawn. It is best to get two people to get the helicopter, and land on the top, and work down, and the other two drive and work from the bottom. You will then have to kill remaining gang members, and if there are four of you, there will be a boat. It is best for two to go after that, and the other two to go after the members by road.
Hangman's NOOSE is where you have to protect Petrovic from the NOOSE. It requires three players. You need to get one of the players to get on the Baggage Cart, and get an Annihilator, then fly over to Petrovic. You then pick him up and the other players and fly to Charge Island. You will become intimately familiar with this mission if you are going for Wanted.
Bomb da Base II involves attacking a Securicar, protected by two other vehicles, driving it back to a helipad in the west of Algonquin, flying it to the Platypus in Broker, and planting bombs in the hold and on the bridge. Always try to drop two players on the roof of the bridge, and have the other two assault the hold. You will then have to jump off the ship within 60 seconds.
Once you complete all of these missions, you should have unlocked Auf Wiedersehen Petrovic. Good luck!