Initially I used the solution in the video above but actually I have found it easier to use the Sofa Car as the opponent's. they wreck quicker and are easier to hit. I managed to wreck more of them in less time.
Here's the settings to use...
Mode: Custom Event
Track: Fairfield County
Car: Bandit Ripper V8 with strength upgrades (any car with strength upgrades will do)
Event Options: Last Man Standing, 23 opponents, AI Vehicle Sofa Car, Damage Realistic.
Difficulty: Automatic, ABS off, TCS off, Stability off, difficulty expert.