Cash For CrashesG 90

You can grind this out by setting up a Custom Event from the main menu.

Here are the settings I used in my custom match to help make this easier to obtain:

Track: Motorcity Circuit
Your Car: Any strong car
Laps: 60
Opponents: 23
AI Vehicle: Lawn Mower
Damage: Realistic
AI Difficulty: Expert

When you begin, simply hit and wreck all of the bots with your car so you're the last one alive. Once you're in 1st, simply wreck your own car to end the race. That should take a few laps of actual driving - but by making it 60 laps you will get a handsome amount of credits and XP.

You can then rinse and repeat this method until you reach level 50.

You can also use this method to help you grind this achievement: