For this achievement, you need to get a 5x multiplier at the top of your screen. This means you need to score Rep for all 5 different colors. The methods to do these are as follows:
Green - Slip Streaming (driving right behind another car) or High Speed
Blue - Drift at the same time as another car or Drive at the same speed near them
Purple - At the start you can get 0-60, going max speed, or you can deplete all your Nitro from full
Yellow - Drifting and Air
Red - Major collision or destroying street signs and the like
It's easy to do these at the beginning of the race (which guarantees purple), but it's very possible to do in a race. Simply follow along behind a driver in your crew and you should nearly permanently have Blue and Green. Wait for a turn you can drift on, then perform the drift and deplete all your nitro at once (it's best to have the lowest tier nitro since there is less to use). Finally, run into the other car or hit a street sign.
You can see my video below for the mid-race completion of this achievements. I had a hard time finding a good race to get this at the start.