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.
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Credit for this goes to Silent Star.
I was following everything as you said but when I would go off of the bigger jump toward the DIG SITE I would do one of the COMBO MOVES and then a B trick on the way back. The COMBO MOVE I did was hold X and then press UP, DOWN, UP and got THE BACON combo. There were 2 bonuses to doing it this way, one was since I was actually doing 3 tricks for the COMBO MOVE my combo meter kept going up and I got to about a 350x combo, the other bonus was I was also getting around 1,500 to 2,000 more points per jump heading toward the DIG SITE then the B trick jump heading back. Its possible to do an X COMBO MOVE jumping toward EGYPT just very risky so I chose to just keep the B trick that way.

Also I would recommend noting in your guide to make sure and not go to far and enter the DIG SITE because it resets your combo meter. I found this out the hard way

This is the equivalent of Skill Rank 30 and requires 638,000 total XP to achieve. Once you've gotten gold on every event, you can grind the final Trick Jam event (Industrial Meltdown) for about 12,000 xp (8,500 for gold, 2,500 for tricks if you're playing well, plus other misc bonuses, I had +10% for being a star in avatar famestar) per ~3 minute run. It will still take quite a while, but this is the fastest method I've found for grinding xp.

Here's what I did to grind out this achievement. Go to the jump that Static Doomsday suggests in the Dunesfield Egypt exploration map. When doing the jumps use combos both ways your score and multiplier will skyrocket.

The combos are the following:
Bacon Strip: +(Hold)
Bang Bang: +(Hold)
The Dragon: +(Hold)
Paparazzi: +(Hold)

What I did was to use Bacon Strip and The Dragon combos on the bigger jump going towards The Dig Site exploration. When jumping back towards Egypt I would use Bang Bang and Paparazzi. Make sure to vary the use to keep them fresh and maximize your gains. By the time I reached multiplier 200+ I was seeing scores of 300k+ and exp values of 3500+ and going up as the multiplier goes up. Keep doing this till you get the exp you need or you get bored (or heavens forbid you mess up your huge multiplier) . Getting to the 250+ multiplier will take some time so be patient and good luck.