Wait for when you spawn in as D-class. Befriend your nearest guard, and ask them if they'd be willing to let you disarm them with their own disarmer. Keep trying until a random guard agrees. If not, you can find disarmers on MTF bodies, or in lockers in armories throughout Heavy Containment (provided you have an Armory Access Level 2 or Level 3 keycard). Hide the disarmer, and use it on an unsuspecting MTF when the chance arrives. This is much easier on RP servers where guards do not shoot D-class on sight.
Disarming an MTF Scientist, or MTF Cadet that escaped as D-class does not pop the achievement, but it works on anyone that spawned in as a Guard, MTF Cadet, MTF Luitenient, or MTF Commander.