well, everything is different if you plan to be a good rapier player or a good phatom player
trust me, the gap was hard to jump for me, and i'm not that good on phantom yet...
whatever, here's what worked for me a few weeks ago :

finish the campain, obtain the Legend trophy, but with as much gold medal obtained in hard as you can, especially time trials or speedlaps.
Winning in hard against CPU is another challenge too, but since it's the same kind of races as online, play online instead.