About the Quest "Doing a dirty job"

Note: quest name may be slightly different

Anyone tried this quest near the Haukke Manor area where you are given some medicine and asked to use it to neutralize Bad Breath from a Stroper and then kill it?

I can't figure out what I'm supposed to do. Tried using the potion out of combat: "invalid target", in combat with the stroper: "invalid target". When the stroper had low hp: "invalid target". After I was hit by bad breath: "invalid target".

Am I missing something here? Or is this just bugged...