I actually got this in Dunesfield (exploration map), to the north of a pyramid on your map, you should see two ramps facing each other "/_\", use those ramps to build a combo, for example, land two different (X) + direction tricks in one combo, turn around jump the other direction doing another 2 (X) trick combo until you get a X10 multiplier, what I did was a (O) + up trick, then immediately did a (O) + left trick, I actually think I bailed but I still got the cheevo......
The best part of this method? NO TIME LIMITS!!!
If you can't find the ramp I was talking about, look at....(Its where he got the 3rd skull)
Solution for Skull Dude In Motocross MadnessEdit: bobbybobinho had a great point about being careful not to overdo your boost, esp if you are jumping north (towards the sea)