I used the Expert sniper rifle for this. In order to equip the armor piercing (AP) bullets, open up the weapon wheel. Navigate to the weapon you'd like to put the AP Bullets on, and click 'RB'. The AP Bullets look like an arrow almost. Now with the AP bullets on equip that weapon. The gun will have to reload with the AP Bullets. Now find an enforcer and headshot that suckahhh. Enforcers have the yellow bar and King like Crown emblem above them.
*Thanks to Moody41 for this tip*
If an enforcer is bleeding out you can still go up to them and finish them off with a headshot, just make sure the bullet is an AP one.