I’ve just finished doing this achievement solo, and I can tell you it’s long and frustrating. Candied Skull’s excellent guide is the most definitive resource but it is no longer available on the web in its original form. Follow the instructions below and you'll still be able to access his maps and videos for the stunt rings. Additionally, I wanted to share some amendments and advanced techniques I used for some of the tougher stunts.
After a lot of searching, I've finally found copies of CandiedSkull's original maps. Please note, I've uploaded these to my personal web space to ensure everyone can see them - I would kindly ask that other solution posters do not copy these links without permission. I would prefer you copy these maps and mirror them on your own web space to ensure the community doesn't lose them again.
The way to read these maps is as follows: start at the yellow dot then then drive along the red line to the stunt ring. You'll need the numbered video from CandiedSkull's original YouTube channel, which still exists through this link:http://www.youtube.com/user/CandiedSkull/
Search his videos for "Crackdown Stunt" once you're at the site.
The notes below are best used once you've watched Candied Skull's video for a particular stunt. All too often, he'll show the best way is to have a co-op partner help, which isn't very useful if you're doing this solo. If I’ve not mentioned a ring below, it’s because the Candied Skull guide worked like a charm.
Firstly, general tips:
Only use the Agency SUV! The B button allows for a bunny hop, which helps you gain altitude when combined with a ramp.
Many of the rings with roofs underneath can be achieved just by getting your SUV under them (usually by creatively throwing your SUV up) and then bunny hopping (hold B then release) into them. See Candied Skull's solution for #33 as an example.
The game will allow you to position one ramp truck and it will stay put until you move it or another truck. Even if you die and respawn, the truck will still be there. However, if you attempt to position a second ramp truck, the first will disappear as soon as it is out of view.
If you have problems with traffic obscuring your run up, wait until nightfall – the roads are a lot less congested then.
Keep track of each stunt you do and keep checking that the amount of rings you've done matches the count shown in game when you complete a ring. If your tally doesn't match, you've missed one. This is very important - a lot of people (wrongly) assume the game has glitched because they think they've done a ring when they actually haven't, especially when they assume they've done a ring because they can't trigger it (it may be they're not doing the right thing to trigger it instead). If you can do this on a fresh playthrough where you know you haven't hit most rings, this can be a big help.
*** So in summary, when attempting a ring: ***
1) Look at the map to find the ring and the correct approach
2) Look up Candied Skull's video on youtube
3) Check my notes below to see if there's anything to add
4) Attempt the stunt
And now for the notes:
#16 & #28
I found both of these easier to throw your SUV and a ramp truck up onto the roof below, then use a different vehicle to trigger the ring. Climb the building and get ready for a very easy jump.
After a lot of messing around, I agree that throwing your SUV up onto the fist is the way to go here. I found stealing a normal truck, placing it in the groove around the fist and then jumping from there to throw made it easier. Once you’re done, follow Candied Skull’s video, and be careful not to drive off the fist!
Don't even attempt using a ramp to get this one, it's easy to get your SUV up there by throwing. Don't try it in one throw, use the surrounding buildings as steps.
It's actually possible to carefully place your SUV on the metal walkway. There's a square area on the right end that you can place your SUV and line it up. Then trigger your ring with a different vehicle and jump up to the walkway. Carefully drive along and do a bunny hop under the ring to hit it.
The only way I could get this was to position a ramp truck at the end of the Agency tunnel. Expect a lot of retries here as you tweak the truck's position to get it 'just right'.
I think Candied Skull may have got the trigger wrong for this one, it seemed to appear for me every time I exited the Agency tunnel. After many tries, I moved the ramp truck about 10 feet left of the location in the video, so that it is on the left sidewalk, still next to the crossing. Best done at night when there's less traffic about.
#34 & #35
Although the logical thing to do here would be to set up a ramp truck each side of the alley, anyone who’s tried will tell you that one of them will vanish as soon as you turn your back. I found that using the truck to go through #34 and then driving up the wall was the easiest way of getting #35. So here's how to do it: immediately after jumping #34 (or going through the motions again to trigger #35), drive through the alley to the side with the 'screw' monument, then turn round. Drive under the ring and use the left wall to reach it: face the wall square on and then apply a small amount of gas; then when you're vertical, hold the B button and add more gas and you should drive up the wall and hit the ring. You don't have an unlimited amount of time to do this, so be careful.
I found this to be a total pain in the neck. Next to the main road that Candied Skulls ramp truck is on, there's a higher road with a ramp each end that joins the main road. I positioned my ramp truck at the eqivalent position on the higher road, just before the road starts to slope down. I then started my run up from under the green sign way down the road.
[EDIT] If you're having trouble with this one (and I know I did), Willkat98 has posted another approach in the comments to this solution. Thanks Willkat98.
I found the best place for the ramp truck to be just in front of the circular piece of road in front of the office.
Good luck and if you're having trouble with a particular ring, leave a comment and I'll try to help.