It's very difficult to beat the ghost(s) of the others players in the ranking because there is only the sixty world firsts for each Grand Prix
You can boost this with a friend. This one must be on your friend list in the xbox live to find him in the filter of the game and to find his ghost.
Ask a friend to do a slow lap in time attack mode. And after when the time/ghost is online, beat it !
I do this with my second xbox one and my son accompt, very easy.