I am not 100% sure about this, but I think that you get the 285 rating by reaching rank 50 on a pedigree 9 chocobo. I've had a pedigree 9 chocobo on two different characters, and in both cases, I got the achievement the very instant I reached rank 50. Both my chocobos had the exact same star ratings though (4* everywhere except 1* in acceleration), so I can't tell whether the star ratings matter or not.