You seem to be thinking of Child's version 3 of Riddles Wisely Expounded, quoted from Motherwell's collection. The refrain is Sing the Cather banks, the bonnie brume / And ye may beguile a young thing sune. The "unco knicht" flies away in a blazing flame in the final stanza.

In the only example of Riddles containing the "Jennifer gentle" refrain, incidentally, there is no demonic element; the youngest sister wins the contest, and the song ends "And now, fair maid, I will marry with thee."

As I said earlier, there's no dispute over the demonic nature of the "hero" in Child 1. In Child 2, however, it is open to very serious question; as I also said earlier, there does seem to be some confusion in this discussion over which is which. You may perhaps have mis-remembered, or have seen a collated text made up of bits of several distinct versions (it wasn't The Oxford Book of Ballads, at any rate). Do please correct me if I am mistaken.