This trophy is incredibly easy. When you unlock the elite level weapon work bench, which you'll need to do anyway for "My Little Fortress", Buy one of the Elite level Sniper Rifles and make your way to a level 3 outpost. Enforcers are the enemies in all red who have gold health bars with a crown over their head when you mark them. But be careful when marking them as they don't stay marked. In the weapon wheel you can switch your ammo from normal to armor-piercing. Select armor-piercing and simply get a head shot on one of these enemies.