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.