With the use of these colorful descriptions, the author is comparing Amalinze, the famed wrestler, to Okonkwo, the young upstart who was finally able to beat him.
Amalinze was known as "Amalinze the Cat" because of his great skill in wrestling. Amalinze was legendary, and for seven years, no one could beat him. The author describes Amalinze's approach to his sport, comparing it to a craftsman who knows his skill better than anyone else. "Wily" means "artful" or "cunning," and Amalinze, the "wily craftsman," had such knowledge of his craft that he was, for a great many years, the undisputed best.
The author goes on to describe Okonkwo metaphorically as being "as slippery as a fish in water." A fish in water, quick and nimble, is almost impossible to catch, and when Okonkwo goes up against Amalinze in the fateful wrestling match, the older man is unable to pin him down. Okonkwo, young, athletic, and strong, is the first to vanquish Amalinze in seven years, as youthful physical ability, as signified by the "slippery fish," trumps age and experience, as delineated by the "wily craftsman." The "fish" has eluded the "Cat" and overthrown him, bringing great honor and pride to the village, and enormous fame to Okonkwo himself (Chapter 1).