This achievement was quite easy to do. I just changed the daysettings to night (=less traffic and a better view imo because you see the front lights of the oncoming traffic), then I grabbed the Gt concept carbon. This car is a fast car but you should be able to handle it. After selecting the car, I drove to the I-88 and started to boost in the oncoming traffic. I didn't press RT the whole time and sometimes I had to press LT, to reduce the speed. The first two times I didn't reach the 20X boost but on my third try it worked and I got a 43X boost. After that I crashed into a toll gate.