you don't even have to leave the north plaza with her bike, just jump off of one of the many ramps laying around in the north plaza

This will work, and it's not a crap shoot like the propane tanks idea.

Pretty much, beat the mission where you have to shoot that bitch on the motorcycle, jump on her bike after it's all done, ride into the Wonderland section of the mall, ride the bike up to the second floor, flip the bitch around and launch off the stairs. Easy 50 points and another achievement.

Go to the parking place where the red convertible and the motorcycle are, make some speed and you'll fly down the ramp leading to the maintenance tunnels.

A good way to get this is by hitting a gas tank or other explosive item while driving. If you do it right, it will send you flying and you will unlock the achievement.