Hello there!
I'm sorry to hear that you have had trouble unlocking "Stinky but Deadly." I was able to retroactively unlock the challenge myself after I defeated a couple of enemies using the sword. The challenge didn't trigger for me, personally, by just equipping the weapon.
Would you be able to try defeating a couple of enemies with the Swordfish equipped as your main weapon? It may take a minute or two for the challenge to update after doing this.
If the challenge still doesn't unlock after trying this, please don't hesitate to update this thread, so we can take a closer look.
Thanks!