This can be done using either your fists (selected from the weapon wheel), or a regular melee weapon, such as a baseball bat or a metal pole.
Rank 2 enemies and above can be found wearing helmets.
When you find one, swap to your fists or melee weapon and hold the right trigger to charge the attack. Aim towards their head and release the trigger to hit them with a charged attack and knock off their helmet.
Later in the game you will start a mission where you enter
. The last enemy in this section is a rank 3 enemy. You can earn the achievement here with one of the melee weapons available.