Here is my Custom Event:
Track: Fairfield County
Car: used the main car with only performance upgrade
Deathmatch 2 MINS, 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).
I got only 30k in pocket when the achievement popped for me, but others reported they got only when reached the total 50.000 CR in pocket, while others got the achievement a day later, so it seems glitchy!
After several updates and reports of other players, it seems now You need the whole 50k together.