After a couple of races in your first MotoGP 3 season, you will get offers from other teams. Keep winning races in MotoGP 3, and you will be offered a contract from a MotoGP 2 team at around race 13. If you take it and keep winning races in MotoGP 2, you will get an offer from a low tier team in MotoGP class at the end of the season. Take that, and this achievement is yours.
The downside of switching classes mid-season is that you will not win the championship in either class and therefore have to race another full season in MotoGP class to get
But it will take a couple of races in MotoGP anyway to achieve
so that is no big deal in practice.