The easiest and fastest way is with the Acropolis level.
At the main menu select Expeditions - Score Attack Mode - The Acropolis level - Tomb Raider Difficulty and as card choose the Wraithskin outfit. You get this card automatically if you buy the dlc. At the beginning of the level move forward to trigger the cutscene and the enemies, run back to the tent and start shooting at the bunch of enemies with the poison arrow equipped. When all enemies are dead reload checkpoint and you will start after the cutscene. Rinse and repeat until the achievement / trophy unlocks (2 minutes)
You can also farm the 50 Wraithskin outfit kill achievement here (5-7min in total)