I'd love to have the recipe. What are your perks into Alchemy as well?
Imp Stool as a base, then add Human flesh, if you cant find human flesh you can substitute it for Canis root, the third ingredient can be either Slaughterfish eggs or scales, or orange dartwing. You can use Mora Tapinella but it will have the side effect of "restore magicka", which wont mean much when your target is dead.
I have perks that improve poisons, aswell as 5/5 alchemy. Also its worth going for greenthumb that collects two ingredients instead of one, and the perk that lets you use a poison for two hits, you effectively quadruple your poisons