I had the game a week before the game came out and got that achievement in about 30 mins. Go into a lobby and boot everyone out besides you and a friend. Just keep racing a short race and you will get 2000 to 1500 credits each time. Very easy.
ya i did what muddy said man its really easy thats y i said not really worth modding i mean i already got like 200k credits if u really want ill upload my save