As stated in my guide for the spike strip hits, I would save this until after you finish the campaign, since you'll have already accumulated some hits, and you'll also have faster cars to make things a little easier.
I did this on the Level 1 event "Just Rampage" since the cars are easier to keep up with, and they seem less likely to avoid the roadblocks, using the Bugatti, which makes it easier to keep up with the lower level cars. I did this in conjunction with the 200 Nudges achievement, since you can get a couple of these before your roadblock powers up.
As soon as the race starts, get a couple of nudges by tapping the lower side of the first car (don't get too much damage to take out the car though because it will lower the amount of roadblocks you can get). Once the roadblock meter fills up, activate it, maintain pace with the car, get a nudge or two, and repeat. You should be able to get 5-10 nudges and about 5 or 6 roadblocks per round using this method. Make sure to finish the round so it counts your progress by either allowing the time to run out, or taking out all 3 cars.
Just a tip on the nudges, try to veer into the opponent with a little force, because it seems if you just scrape the side or don't hit with enough force, it may count as a Grind instead of a Nudge.