Heist:
Any (I did Mallcrasher)
Difficulty:
Overkill or above is recommended
Weapon:
Kunai Knife (Melee) - Unlock level 54
Armor:
HeavyOther Equipment:
Concussion Grenade or
Shuriken - Unlock level 25
Load up a heist on overkill or above and get a police assault to begin. Once it begins, get ready for a big group of enemies to appear, about four or five most likely. Use a flashbang to stun them and then go ahead and stab each of them at least once.
If there are still enemies standing after everyone has gotten stabbed, continue to stab them. There is a chance that the Kunai does not poison enemies, however it appears that chance is quite small.
An alternative method could be to use the Shurikens to inflict poison. On lower difficulties, they cause a lot of damage and will more than likely kill the enemies quite quick. But Kunai paired with Shuriken on Overkill should net you the achievement without much trouble. The bow and arrow (from western DLC)with a poison mod is also an option.
UPDATE: From
z RagnaroK z's comment down below, in regards to this guide:
Your guide is outdated. As of March 2018, they have changed % for the poison items. The shuriken star is now a 100% chance to poison. while the Kunai is a 75%, and everything else is 50%.
*If you give me a negative vote, please let me know why so I can improve any future solutions