Crackdown 2

While the other solutions are good, I've got another pretty easy one that I used. First off, extract yourself to this location:

(This is actually an agility orb map, but the extraction location is right around where the circle is).

As stated in the achievement description, you must do this with an Agency SUV, so extract yourself to the location with the SUV. Pick the SUV up and take it to the top of the gas station that is right beside your location and point it towards where the helicopter dropped the SUV off. Then extract yourself to that same location, and quickly get back up to the SUV on top of the gas station and get in it. Once the helicopter comes by to drop off your other extracted vehicle, just drive off the edge and into the helicopter. Make sure not to use the jump ability, as you'll land on the helicopter's blades and won't get the achievement.

A much simpler way to do this by yourself is to use the Store Vehicle option at a drop pad. First, call in an SUV. Next, move it out of the way they find a small car to store. Put it on the pad and call for the chopper. Once the chopper is only a few feet off the ground, drive up and jump into it.

You can do this on your own. Just call a helicopter in to drop off a vehicle, and have an SUV in position so that you can jump/ramp (remember, the SUV can jump with the B button) into the helicopter when it shows up. If your strength and agility are high enough, you can pick up the SUV and jump up to a ledge near the drop point.

The first time I tried this, I ended up jumping into the helicopter blades, destroying the heli and not getting the achievement. You have to have the body of the SUV hit the body of the heli.

I got this at the northwestern-most tactical location on the southwest island. I actually found a ramp truck (parked next to a nearby gas station) which made it very easy to hit the heli from below. I positioned the ramp, set up my SUV, called in the heli, and as soon as it dropped the vehicle, I hit the gas and gave a little jump off the end of the ramp. Bingo.

The easiest way. Have the helicopter drop off an SUV. Grab a car nearby. Store it. When the helicopter comes, press B and jump into with the SUV. Can't miss and you don't have to go through the hassle of using the ramp truck.

This method requires two players

Someone will have to have Driving level 4, and someone will need Agility level 5 to get the helicopter.

Someone with Agility level five must go to the Agency tower and steal a helicopter from the helipad (you will have to scale some of the tower and then glide to the helipad). Once you have the helipad, have someone call the Agency SUV at a tactical location. Meet each other at some street in Pacific City and have the helicopter just hover at a low altitude. The person in the Agency SUV should drive at the chopper and use the B button to jump into it. When jumping into the chopper the players should note that if the SUV touches the blades of the chopper, both vehicles will be destroyed. As long as both vehicles touch, the SUV driver will get the achievement.

Afterwards you can switch vehicles so both of you can get the achievement. Just be careful when landing the chopper to let the landing gear extend before putting it on the ground, otherwise the chopper may become unusable.

(A little joke for people who have seen Live Free or Die Hard) Killing people like this is only recommended if you "Ran out of Bullets"

This can also be accomplished using the Portable Launch Pads from the Toybox DLC.

Select a drop point in Unity Heights, preferably one with a nearby building to place one of the pads (northwest on the middle island worked for me). Dash out quickly and make note of the distance of the helicopter for your trajectory. Place the pads, and either store a vehicle or request a new one (I requested another SUV, since it would be precisely the same drop). Drive into the pads, and presto!

Just like Spartan's solution, mine requires two people, however it's a bit easier (I think). You need one person with driving level 4 and you need to have completed the Hope Tower Tactical Location. It's across from where Shi Gen tower was.

Anyway, right behind the resupply point is a ramp that leads up to the doors of Hope Tower. The first player calls in the agency SUV and drives up the stairs to the platform overlooking the supply point, he should then align the SUV with the supply point. Once that's done, the second player calls in a vehicle drop. Once the helicopter is low enough to drop off the new Vehicle player one drives forward and jumps with the SUV to land atop the helicopter.

Rinse and repeat for player 2.

If you're fast enough this might be possible with only one player, I'm not sure. A mate and I figured this out and did it this way. Certainly it seems possible to do this solo.