Power Outage - S3-E7
Corrected entry: Prue says that they should practice their demon counter attacks so that they can get a piece of Balthazar's flesh, as it "might be helpful for the potion". But in every future episode they say that the flesh is necessary for the potion.
Correction: It was their first time making that kind of potion. They knew in the future that is was the only one that would work.