Poor techniques (compared to the Greats', at least; they're mostly effective against sub-Great fighters), cowardice, and generally being an idiot in fights. A recipe for failure if there ever was one.
I kind of take the "doubled his power" thing with a grain of salt, and feel that Jin Yong might have meant it more figuratively (and hyperbolically) than literally. None of the existing Greats doubled his inner power during that same interim, and yet, they weren't exactly grossly weaker than the monk in inner power even after the sixteen years.
Pretty well. I think each one has a decent chance of winning.