There are alternate methods noted in other solutions which require you to have the DLC Agency Buggy or a friend with the game to throw you in the air. This only requires 4 star driving skill (which you should get anyway for the awesome cars) and is much more satisfying to complete.
The text guide below is the same as this video guide, so if you prefer you can read that.
You will need to use the 4 star SUV and head down tunnel 2 toward The Den(Volk). Continue through the tunnel and try to aim for 130 - 140 Mph (210 – 225 Kph) at the last corner. Once round, accelerate hard. You want to be at 150 (250 Kph) at the end of the ramp.
One thing to note here is that using too much speed will make rounding the last corner difficult, making you lose speed on the up ramp. Try letting off the accelerator briefly at the turn. Also, you will find that going too fast will make you hit the exit arch and bounce straight toward the floor, so just take it steady and keep it smooth.
Use your hydraulics for extra height and push forward to put yourself in a front-flip. Make sure you get 2 flips. The game will not display it but it also counts a long jump as one of your 6 stunts, so assuming you went far enough you will have 3 out of 6 and will only need to repeat this jump once more.
Go back halfway down the tunnel and do it again. Hopefully you should have your achievement. This is how I got it anyway.Edit: video re-created in glorious HD!