I was actually thinking it looked that way at first, but when I rewatched a part of the ep before my previous post, the episode itself has Jellal commenting under his breath that he can't beat someone like Jura using borrowed magic. Such a comment opens a door wide to the interpretation that it's exactly because it's borrowed that he can't employ it properly to beat high level opponents. Of course another interpretation is that no magic but his own is good enough for the task, but after everything Jellal has gone through, including getting beaten, I doubt he's prone to underestimating other wizards' techniques too hastily.