The good thing is that you can race in any Tier using any unlocked car. Unlike the Xbox version of this game, in which your choice of car was limited to the particular tier. So if you're having trouble with any of the races, just come back to it in Pagani Zonda when you unlock it later.
I got this achievement with the PORSCHE GT3 RS, which you unlock at level 9. I managed to get three stars in some races at level 8, but I had no trouble with any of them with the PORSCH.
So don't worry if some races don't seem doable at your level.
My tips are:
- Try to get one or two stars in all the races, don't re-do them even though you think you might get a little bit better. (I tried one of the speed trap races like 20 times but all I could get was 2 stars. Then with a better car it was a piece of cake)
- Always use a new car as soon as you unlock it. New cars are usually better - improved speed, acceleration or handling.
- Re-do the races that you're good at to get more bounty (XP). A race you've already done will give you less bounty though. So re-doing races you only get one star in won't get you very far. New races are always better.
- With a good car you can afford a few mistakes and still ace the race.