This is easily done in one of the longer race levels or in sandbox mode. Lean back into a manual with the and double jump with and keep the held back to do a backflip. If you want to rack up your multiplier quicker you can always tap the button while facing up to get more air and hit and to do tricks to further increase your multiplier. Just repeat this until your combo gets high enough then land it. The longest race in the game is the "Big Boss" race in "Director's Cut". You should have no problem getting a 100x combo before you finish the level. Alternatively if you are feeling lazy you can load up an empty sandbox level and push the left to manual then push the right to endo. Your combo should go up 1x if you rock back and forth fast enough. No jumping required! The achievement doesn't often unlock right away. Sometimes you need to play through another level before it pops up.