Yeah the do all tricks trophy is hard too. But u might wanna get the list from the xbox360 site. Then you do it trick by trick on sandy mountain. Start with the triangle tricks as soon as possible.
The Killer combo is pretty easy. Go to Phu whatever level and after finishing the first round right at the first jump do a special then at the small ledge in the turn to a quick x-trick to keep the combo going fly over the following gap best with up+triangle then sharp right next special immediate boost several circle tricks. Sounds easy huh?
@ultimate: In my opinion the higher the trick rating the better ur chance of landing the two-timer. But I can follow Clicks argument too. Best u try what works for u.