This is made very easy if you have a second controller.
Start a split screen race. I chose The Farm, as I found the shield spawned more in later levels. Drive around with your main account until you pick up a shield. Then with your second pad, go pick up an Uzi.
Once this is done, line the second account up behind your main, activate shield and unleash the Uzi on it. It pops almost at the end.
Job done and 25g is yours.
This should be easy enough to follow, but if there's any problems let me know.
According to Jayour: Seems as though the Uzi acting as 20 hits has been patched out of the game. Would be nice if you could add this to the solution.
The quickest method now seems to be using player 2 to unleash melee hits.
I can’t confirm this as I already have the achievement.