In "The Cask of Amontillado" what is Fortunato's one weakness? How does Montresor use this weakness for his revenge?

The way that Montresor lures Fortunado down into the catacombs is by playing on his fondness for wine, and for thinking himself an expert in wine-tasting.  Montresor states, right at the beginning,

"He had a weak point—this Fortunato—...He prided himself on his connoisseur-ship in wine."

So, under the false pretense of having a great cask of Amontillado wine that he paid a heavy price for, he approaches Fortunado.  He tells Fortunado of the wine and that he is worried that he paid too much for it without consulting Fortunado's expertise.  Then, through careful manuvering, he gets Fortunado to insist that Montresor take him down to the tombs so that he can taste the wine and determine if it is truly a good buy.

So, Fortunado's weakness is in wine, but it is truly in the fact that he prides himself on being a wine expert.  And, Montresor plays on that self-pride.  He wonders aloud to Montresor whether he should have consulted another guy, Luchesi, on whether it is a good wine, to which Fortunado insists, "Luchesi cannot tell Amontillado from sherry," and so deems himself the only man capable of determining if the wine is authentic and quality.  Montresor is a cunning villian, in that he plays on Fortunado's vanity and pride to lure him down to his death.  I hope that helps a bit; for your other questions, I suggest submitting them one at a time, as the guidelines of this website allow for one a day.