Following the strategy on this page, I was able to defeat him at level 85 (but I'm sure that with the same strategy, it could be done even lower than that.)
Disclaimer: I shot two versions of this video - the first one, I recorded commentary but hadn't turned on the "microphone" setting in Game Recorder, so I got no sound.
The second one, I recorded the audio, but when I played it back I found out that the game's music was too loud, so you can't hear my explanations anyway. So, here's the first video (without sound) - but hopefully by watching it you'll still be able to see this strategy in action:
(P.S. Battle starts around the 2 minute mark.)