Crackdown 2

If you have the Toy Box DLC, you can use portable launch pads to solo many of the stunt rings. Position the first at the end of your jump trajectory, and the second where you need to launch from. You can then drive any vehicle over the launch pad.

One note for everybody before I get a billion negative votes. If you don't want to try driving through the stunt rings, you can be thrown through them by a friend (like in Crackdown 1) or you can mag grenade your vehicle to the helicopter and have a friend fly you through them.

I did the magnet grenade on helicopter and car method. As the helicopter pilot, here are a few notes.

There are two or three of the stunt rings that have four total rings that you must pass through. You can only see three rings as the helicopter pilot. This will require communication between you and the individual in the car.

There are also a few stunt rings under bridges. You can still use the helicopter method. Go to either side of the bridge, and position yourself so the helicopter is above the roadway, and the car hangs below. The magnet connection will not be affected by the roadway. You will need to go slowly as not to damage the helicopter too much. Again, since you can't really see the ring, have the person in the car direct you.

There is one ring in an alleyway between two buildings. If you go slowly, you can move through here and your helicopter blades will pass through the buildings without wrecking you. Do this SLOWLY.

This should cover all the specifics if you co-op the stunt rings with the helicopter and mag-nades.

Edit: Thanks to CadmusTheDragon's comment, for this following tip.

Using heavier vehicles will cause the Mag grenades connection to become longer. This alleviates some of the bridge issue above. And, using larger vehicles will make it easier to hit rings, as their is more vehicle to hit the rings.

As others have mentioned, all the multiplayer tricks from Crackdown 1 are still usable and C2 adds the "hang-from-a-chopper-using-mag-grenades" method.

However, all of the stunt rings are fairly do-able normally as a single player, and IMO easier than in Crackdown 1. All but about 5-6 of them require you to use the Agency SUV, and use its "Bunny Hop" capability. Most of those involve getting up a good running start towards a game-provided ramp somewhere (there's ALWAYS some sort of ramp provided) and launching the hop just before you get airborne. After you've done it a few times, it becomes second nature.

The most difficult ones are in Green Bay, on the beach and at the broken ferris wheel--in both cases you need to start WAY back to get enough speed up to clear the last ring without falling below it. That makes it tricky to line up the jump, especially on the sand where the rolling terrain tends to bounce you around a little. For that one, press and hold the "B" button just before you hit the last sand dune before the ramp to engage the "Super-Grip" over the last hill to prevent you from being bounced off-course.

Another difficult one is in Green Bay (on the "other", northern island) just in front of the stadium because the rings are positioned at an angle that prevents you from getting a good running start. But there's a Ramp Truck right there that will help (although it can be done using the rocks that have fallen on the road and without the ramp truck). I found that I was able to hit the first two rings using the rocks as a ramp, then use the SUV's "Super Grip" ability (hold B without releasing) to drive up the wall as if to climb onto the nearby overpass. Part way up the wall I touched the third ring and completed the stunt.

There are two that are launched from the top of the parking garage in Green Bay. They are fairly straight-forward, but tricky to line up AND get enough speed, because both require you to make a quick turn at the last moment before launching.

The rest I found fairly easy. If it's a single ring, look for the closest, highest place to jump at it from. You might need to carry or throw the SUV up onto the top of a building or hill to get there. If it's a multi-ring set, then look for a suitable (high-altitude) place in the opposite direction of the first ring to launch from. And remember that most of these allow you to miss the last ring on your main jump, but immediately turn around and do a Bunny Hop to catch it within the time limit.

Finally, keep in mind that there's one set of rings in Unity Heights--on a broken bridge--that you might want to hold off getting until after you've gotten the "Party Bus" achievement.

I've just discovered another way to do this while helping out a friend. Two people are required, as well as owning the Toybox DLC. It also helps if both people are using agents who can see all the rings, oddly enough ;)

This is actually just a variation on the helicopter trick, although this way is profoundly quicker. Simply use, instead of the helicopter, the Mass Driver; it allows someone to target and pick up a car and, as long as the trigger is kept down, held floating in front of them.

Just make sure you hold the car close to the ground so the second person can climb in. After they're in, simply rocket boot your way through all the rings. Although it should be noted that a handful of the rings are too far apart, and the timer will run out before you can pass through both, those few tend to be easily done by other means.

It took less than half the time to help my friend doing it this way, than helping another do it with the helicopter.

Just don't bother trying to fire the agent-filled car through any rings (or at any renegade driving orbs), the agent's weight cancels out the force of the throw leaving the car to simply drop to the ground.

There are many different ways for getting the achievement. You have the traditional method, which is using the ramps provided by the game (the rings always line up to some kind of inclined surface, from road ramps, ramps made of rocks, stairway ramps, wooden ramps...), and an Agency SUV. Traditionally, you build up enough speed on the Agency SUV, and when you get close to the jump, hold down the B button to grip the ground, and letting go of the B button at a precise moment as you reach the jump, and you'll be launched towards the stunt ring(s).

There's the launch pad method which requires the ToyBox DLC. You simply place some launch pads and create a path through the stunt ring(s) and then drive over the first launch pad into the ring.

There's the thruster method which also requires the ToyBox DLC. A friend who has thrusters and can preferrably see the rings (or have good communication with you) lifts up with the thrusters and throws you (while in a vehicle--the larger the vehicle the more space you cover) through the stunt ring(s).

There's the magnet method which requires you to have mag grenades, and use them to link a vehicle to a chopper, preferrable a truck or ramp truck, as the increased mass of the object allows for a greater link distance between the chopper and the vehicle. Then the pilot flies the linked vehicle through the appropriate ring(s).

Communication is a must, and having another player that also needs rings helping you is preferred, as they can see the rings so it's easier for them to navigate you through them. Then you guys can take turns and use communication when you're getting them through a ring you can't see, but since you just got it, you'll have a better idea of whereabouts the ring(s) will be.

Some rings are only 1, but others can be sets of 2, 3, or even 4 rings (where you couldn't see the 4th normally until it's activated) where the sets need to be passed through sequentially within a short amount of time. For the set ones, you should be quick about it, as they can time out, which means you'll have to start from the 1st ring of the set once again. Unity Heights has a couple of challenging ones, like one where it's above some stairs and another at the upper edge of an archway through a building, so be absolutely careful to line everything properly when using the magnet method, and also try to implement physics (like for the archway one), where you swing the linked vehicle through the ring so you don't crash into a building.

If you're using the helicopter magnet method, note that the link will not break through objects, so if you have a ring just below a bridge (as there are some like this), the helicopter can fly low over the bridge while the linked vehicle goes through the ring below the bridge. Always take risky rings like these slowly, so not to cause an explosion via friction.

Ramp trucks are ideal to use, so I've attached a picture and video of the location of one of the first ramp trucks you can find and then store (at a tactical location) to retrieve at a later time.