First off, hat tip to Telexen0 and his solid solution; however, I did want to provide an updated solution that reflects the current situation online. Hat tip also to Manifest95 for his boosting sessions, where this method was developed and refined.
To get this achievement, you have to complete 50 level-3-to-level-4 upgrades in the Blinged Out Ride game mode. To upgrade your car, you need to acquire money, which you can do either by killing your enemies ($100 each) or by destroying the enemy team's car ($500 each). Note that teamkilling people or cars does not drop cash; only killing the opposite team works.
It costs $500 to upgrade from level 1 to level 2; $1000 for level 2 to level 3; and $2000 from level 3 to level 4. When a car is destroyed, it reduces one level -- as you will see, this is critical to know when boosting this achievement.
I generally recommend assembling a group of 8 boosters to work on this. 6 will work, but you may have trouble starting a match without randoms. 10 or 12 will start faster, but you may have issues matching up and it may stretch out the time involved.
First, divide the boosting group into two groups of 4. Each will want to determine a host, have that person set up a party using the in-game functionality under the Xbox LIVE menu, and invite the other 3 members of their team. The party system allows the boosters to play on the same team in the same game. The game will not start without uneven teams, so do not worry if you get a single random, they will be kicked when the match begins.
Each host will want to simultaneously search for a Ranked Blinged Out Ride match. It is helpful to assemble all boosters into an Xbox LIVE party, as the matchmaking does not indicate who you are paired with. I recommend that the hosts tell each other how many people are in their matchmaking lobby, to confirm that they are together. Once you have a match with both parties and only 8 people, wait and eventually the match will begin. (I do not recommend playing this gametype with randoms. They can, at times, be managed, but they have a habit of teamkilling, blowing up the team cars, and generally getting in the way and ruining the rhythm.)
I recommend establishing a boosting order. The first person to get upgrades should immediately take the car to the upgrade shop (follow the green wrench indicator); the last person should drive to the opposing team's base to start killing at the spawn points (recommend to use the AK or SMG); the other two team members will stay their base and die. The dying boosters should collect the cash drops, which will allow the upgrader to upgrade the car. $500 and $1000 are pretty quick to obtain; I do recommend that the upgrader return to base to help get to the $2000 for level 4, by allowing themselves to be killed as well.
Once the upgrader successfully gets the car to level 4, they should drive the level 4 car to the opposing team's base. At that point, the other team should blow up the car and kill the previous killer. The former upgrader becomes the killer, the former killer respawns at the home base to be killed, and one of the people being killed becomes the upgrader. The blown-up car will reduce to level 3 and respawn at the home base.
Continue this cycle until the match ends.
This method will generally allow each member of each team to get about 2 upgrades per match. At 20 minutes per match and 25 matches required, it will take you over 8 hours of in-game time in even the best case scenario (no randoms, no disconnects, etc.) Given the horrid matchmaking system in this game, you should expect to spend a few additional hours just getting into matches.
I hope this solution helps; please let me know if you have any questions and I'll do what I can to improve the solution.
Good luck!