Burnout Revenge

There will be races, starting after you get into the Fearless Ranks, that will enable you to use Crashbreaker after you either wreck or get taken out. The are called Crashbreaker races.

Unlike the Crash events, you will only need to hit the B button once for it to detonate you car.

You will need to be leading a race early for this to be most effective. Once you crash and you go into Aftertouch mode, wait before you activate Crashbreaker. Wait until the next car starts to pass you and you see some other rivals close behind. The blast should take out multiple. Hopefully the will be 5 grouped together and the achievement will pop.
The top left hand of the screen will show you how many you have "payed back."

Notes:

None of the racers need to be "Red." They mean "rivals" as in other racers in this achievement.

Its best to be in first place when you crash.

Make sure you wait for the group.

It is done easiest early in a race when everyone is still bunched up.

It works best in sections where there are only 2 lanes and no shortcuts.

There are many races to do this in. You may not get it on your first couple of races, but you will have plenty of opportunities during career mode.

I tried the other solutions, The one for over a hour and had little to no success with either, then I stumbled across a little video on youtube, courtesy of achievement hunter.

Load up Insane > Sunshine Keys > Crash-breaker race reverse (last available race)

From the start of the race get yourself some takedowns so your crashbreaker is up to x4, then get ahead of the pack into first position look behind and have a little crash, as soon as you see the pack coming detonate your breaker and with the tiniest bit of luck you bag all five,

Took me two attempts, 5 minutes.

The video is below, again massive kudos to achievement hunter, really easy when you do it this way.

I good way I done it a few times is when you unlock the FEARLESS rank on WORLD TOUR go on ETERNAL CITY and pick the Crashbreaker Race-Long Reverse and pretty much for the hole first lap all the other cars will huddle together and this will give you a good chance to get it! But make sure you have lots of boost because that is what you need to cause massive damage to the opponents around you! All I kept on doing is making sure I was first with a good amount of boost , looked back to check all the other opponents were behind me and then crashed into the nearest wall and hoped I would get them all (After-touch takedowns don't count) A place I kept getting the total payback is the archways. Crash into one of them and usually all the other cars will head into the archway , wait for the right moment and then EXPLODE!! If you Don't get it the first time then just keep restarting the track. I hope this has helped some people... Good Luck! :D