Here is my Custom Event:
Track: Fairfield County (any Arena good)
Car: Lawn mower
Deathmatch 20 MINS (quite after achievement pop), 0 opponents, the rest not important
At race start just go forward with full throttle to the wall. And again... 1 wreck is around 10 secs, so max 17-18 mins for 100 Wreck, but I'm pretty sure you already had some! :)
FYI: You won't get any points, so just quite after the achievement. If you want exp + CR, you can add opponents, but that's way slower and they will just bother you to wreck yourself.
You can get this too:
In this case just wait until the time runs out and restart. After 15 mins the controller turns off if not in use!