The stage Paskuri has a ton of jumps right at the very beginning. Make a Custom Championship there (hardcore damage off) and take Group B (4WD) as your car class. Then get in your trusty Audi from that super fun online achievement in the base game, or whatever car you happen to have in that class, and go as fast as you can through the first section. Chances are whether intentionally or unintentionally if you just keep the pedal down you'll flip it and as long as you don't get terminal damage you'll get this achievement:
Then just keep running the beginning part flat out until you crash and restarting and you'll have your 100 jumps in no time.