Ok for this achievement you are required to have the highest wanted level in the hub.
So for me the earliest opportunity to achieve this straight after I completed Level 2, I was then sent back to the hub area are Malcolm Marlyn & Your Custom Character.
If you follow the ghost stubs until your up on the bridge area as shown in below map as P1 (player 1):
There will be a truck with Silver doors and a Police Officer stud in front of it.
All you have to do is shoot and kill him with
to be awarded the highest wanted level which is 3 shields (bronze, silver & gold) above your characters head.
If this doesn’t work for some reason smash up 3 civilians to explode within a short space of time to get the 3 shields, and you will be sure to be awarded it.
Then instantly you will hear that magic pop achievement unlocked.