One championship, one world Trophy in MXGP2 - The Official Motocross Videogame
One championship, one world
Complete a multiplayer Championship
How to unlock One championship, one world
These require 3 races to complete. I do recommend combining them with the Stadium events as outlined in the walkthrough to save you some time. There are 4 events in the Stadium Series, but you will need to win all 4 Stadium events anyway, so why not group them.
Setup a championship in multiplayer ---> Stadium Series Championship, No AI.