This achievement is amazing for two reasons. First, Amalinze the Cat was the greatest wrestler in Umuofia, who "for seven years was unbeaten, from Umuofia to Mbaino." So in all the neighboring villages he was known for his skills in wrestling, & even grown men could not defeat him. Second, Okonkwo was only 18 when he "threw the Cat," which earned him the respect and praise of the Ibo villages. The wrestling matches were feats of strength and endurance that tested the virility of the men in the tribes. For a man as young as 18 to defeat the greatest wrestler was an achievement most could only dream about. But Okonkwo did it, & in doing so, established a reputation for himself. It certainly wasn't easy- the fight is described as one of the fiercest ever, with muscles standing out on backs & arms, where "one almost heard them stretching to the breaking point."
This fight solidifies Okonkwo as a rising star in the tribe, & puts him in an enviable position to earn many titles. It was also a point of personal pride, as it proved to him that he was nothing like his father Unoka. Unoka is often described as weak & lazy, & Okonkwo successfully distances himself from that image by throwing the Cat.
Okonkwo's defeat of Amanlize the Cat was considered amazing because Okonkwo was just eighteen at that time. Amanlize the Cat was considered the most strongest wrestler at that time, he has been undefeated in all the villages for seven years. So when a new wrestler comes and defeats Amanlize the Cat, who has been undefeated for so long, Okonkwo becomes powerful and respected throughout the village.
Okonkwo defeating Amanlize the Cat was such a great a feat because for 7 years Amalinze had been undefeated in all of the 9 or 7 Kingdoms. Also, when Okonkwo fought Amalinze he was just 18 years old.