DiRT 2

Ok so for the best part this achievement is quite hard as most of the time you'll get in a race and on the first corner you'll be planted into a wall by your opponents or you'll be up against some DiRT enthusiasts you know how to race every track perfectly.

So your best bet is to search for a single pro tour (as team wins dont count, unless you still place first overall) on rally or trailblazer as these game types are non contact (opponents are ghosts) domination also works quite well as it goes on your time.

There will be tough competition so you're gunna need to get used to the game and unlock some of the special cars like the dallenbach (trailblazer) before the wins start coming in.

Ok so after finally getting stuck in with the online myself i can suggest some good vehicles to use (personal preference)
Rally: Subaru Impreza Sti Group N
Rallycross: MG Metro 6R4 or Ford RS2000 (if you can handle it)
Trailblazer: Dallenbach or Nissan 350Z
Buggies: Herbst Smithbuilt or PRC-1
Stock Baja: Hummer H1 or Honda Ridgeline
Trophy Truck: Toyota Stadium Truck
Raid T1: Mitsubishi Pajero Dakar 1993 or Hummer HX

Thanks to aaron451 for the team win correction.
If iv missed any game types that make this easier feel free to comment, good luck.

This soultion will be 10X easier to do then the top soultion

This is a method to boost it with 2 people (1v1 double boxing)

How:
Now at the multiplayer menu hit pro tour and go to the Ruckus section now you will be in a lobby. With the lobbies the reason it is very easy to boost is well nobody plays the time specific modes they want to just race tracks and thats it. When your in the lobby check what mode it is (read the ticket thing at the tope and right beside the game mode is what you are looking) if it is domination get both of you to quit out until someone finds a last man standing.

Why its easy:
The person who is going will obviously win the race but since its timed its based off that so who ever wins leads the race til everyone is eliminated which doesnt start til a minute in (if there is only 2 it should take around 1m.20s.12ms)It is very easy as this dead mode is very easy to boost in playing legit obviously 3 things to avoid:

1) The Dallenbach is always going to be first even when i drive it i beat people (im not good myself!)

2) Watch the levels whos going to win the 1's or the few guys with the 300's ?

3)The Break button is your friend dont be afraid to use it