Screw rookie. Be a man and play in all time, and I prefer MX Battle Grounds for a tack, its really shorthow to reach level 50 fast :
1.Go to single player (well choose bike or quad of your choice).
2.Go across to "Short track" click on KINOKO.
3.Press A on "Opponent skill" then move upto rookie press A.
4.Press A on "Duration" go to 5 laps press A.
6.When finished race (When you rider is on the podium) press Y to restart.
and do this until level 50 (the highest)or until really bored (must admit it does get boring).
Thanks Hope I Helped any Of You Guys.