Motocross Madness

What I did was play Egypt on the first stage of exploration, I went to the double jump that goes across the river ( Just head straight from spawn past the first pyramid and it will be on you're right, clear as day) ,from there I kept doing X tricks back and forth one side doing 4 tricks and the other 3 because the ramps are a different angle. I got my multiplier to 250X (This part takes about ten minutes, granted you don't screw up) once there you will already be on fire just do one B move each way and continue to do so until you earned the desired exp, I was earning around 10,000 Exp per 10 seconds with this method.

To get this achievement really quickly simply set up a spilt Screen trick race and choose the very last track now exit out and now this track is unlocked to use in single player. On this track in trick mode a gold medal earns a minimum 6000exp plus whatever bounses you also get plus you also get $6750 which helps with the Rolling in the dough achievements for earning $100000.

credit to MrZombieChicken for pointing out the trick of unlocking tracks in Spilt Screen

You need 247,000 xp to get to level 20. I got gold on all races but 2 my first try and was level 16 when I got the achievement for all gold medals.
To get the xp you need to get level 20 from there, replay Iceland Trick Sessions or Rival events because they are the shortest and give the most xp.
If you wait until you have all gold medals, you will have less to grind in the end and they will be easier with a maxed out bike.

I chose to do Geyser Field in trick mode after I got all gold and got over 7,000 xp for getting 100,000 points in a run each time.
From level 16 it took me 14 runs of this track at an average of 6,500 xp each time.

I got this achievement by playing the game as normal and got to level 19 and then I repeated stoppies to get to level 20. Even a small stoppie can get you a large amount of XP. Stoppies can be performed by maintaining a slow speed whilst tapping the brakes and holding the bike forward. This can be a bit tricky at first but once you've got the hang of it, it's a quick, fairly easy way of gaining a lot of XP in a short amount of time.