Ben 10 Galactic Racing

Pick single race then "Primus Dominus" circuit then "Havoc Highway".

At the first jump right after you start use your right joystick and press up for front flip down for back flip right to roll right left to role left in any order but you cant use the same stunt twice or it won't count for your combo.

The combos do not stack if you get this achievement you will not unlock your 3X or 2X combos but you will come to another jump later (the last jump before the end of the lap you will jump through a circular rock and there are 4 spinning circular light platforms below) this jump is only big enough to do 3X combo but if you don't already have it you can get the "Triple Threat" achievement here for completing 10 3X stunt combos. This is also a good level to get the 2X combo as well because almost every jump in the level is big enough to do 2 stunts.

If you are visual, it's the exact same thing as wbs 101 said :

(You can restart once you did the 4x Combo*) ;)

*Credit to x360a.org*