Career Challengers Championship 1st event is a race with a bus. If you finish the challenge (wreck 8 car) you get that car. You can keep trying this race. After start wait around 30 secs, so the opponents start coming in front of you. Higher speed, higher hit damage. Hit as many as you can keep going forward, at the end, at the corner maybe you can hit some more. Turn back, wait again a bit (and go), so they will come again in front of the road. Do this until the end of the race. Hope this help.
Or if you have the bus make a custom event:
Track: Fairfield County
Deathmatch 2 MINS (see below), 23 opponents, AI Vehicle Sofa Car, Damage Realistic
HARD difficulty, Automatic, ABS off, TCS off, Stability off, difficulty Expert.
With 2 mins you can easy wreck 8-10+ cars and get 18-20K EXP points and 2.640 CR (FYI with 10 mins setup got 50 wreck and got 41k and same 2.640CR).
UPDATE 2021.01.21: As Nord1cB3ar mentioned maybe The Supervan is a better choice right now. Maybe because an update or maybe else.
I don't have the game now, so I can't check. When I wrote the solution the Sofa car was the best option, because the "one hit kill".