The best time for this is during the main story missions towards the end of the game.
I took a poisoned sword and bow just to sure it up.
There's a main mission that takes you to Cyrene. There are four guards at the top of the Akropolis all level 35/36. Sneak up on one, hit them with your bow (preferably) and hold
to bring out your torch and hit him within 30seconds.
I did this three times before the achievement app realised I'd done it, it actually unlocked during the scene after you confirm your main story kill, during the cutscene.
If you elect to use you blade instead, only use RB as RS will take their life down so far that the poison may kill them before you get your torch out.
You can also do it the way my friend did it.
Kill a guard on the way up to the Akropolis, put him somewhere close to a hide spot out in the open and poison the body. DO NOT get your torch out too early. Wait for a guard to come over and inspect the body, as he does and gets poisoned, leap out with your torch abd smack him on his head, yelling, "Bad Mr Kitty".