This can be done in "Kyrenaika" territory (northwest corner of the map). In the town of Cyrene, there are Level 35+ enemies walking the streets. You can easily find them using eagle Senu. You need the "Poison Darts" skill. Make sure to equip the poison darts in the gear menu afterwards. To activate them, hold D-pad Left and throw them at an enemy (must be Level 35+). You will see some purple fog around the enemy, indicating that he has been poisoned. Then, quickly kill him with the torch (hold D-pad Right to pull out torch). It is recommended to wait until you are Level 35+ to do this since you inflict more damage to the enemy and he dies quicker.