You need to finish on the top 3 for each of the second three sets of races to unlock the 200 HP Championship. Then, you need to be on the top 3 overall ranking for that championship for the achievement to unlock. This second championship is just a bit harder than the first one. However, if you have trouble getting in the top three, my advice is go to time trial and learn the tracks (shortcuts, shorter detours, tricky curves or jumps, whichever works best for you) & become familiar with the car you'll be using to race (faster cars are usually more unstable & you end up crashing a lot or maybe land facing backwards, so you may do better with a slower car that's more manageable, like the truck category). Even if you are familiar with the tracks from the 100 HP Championship, 200 HP cars go faster, so you might need some practice to master the track at higher speeds. If you still can't get in the podium, you can also try finding car parts to unlock better cars (you'll then have to buy those to be able to use them).