is Mcmurphy "A Napoleon, A Genghis Khan, and Atilla the Hun"? Why?

You can make a case that he could be a little of all of these. The use of these allusions suggests that McMurphy tries to conquer people/his environment (including Nurse Ratched) like Napoleon who believed that he could achieve whatever he wanted regardless of his size (think how much smaller than Chief Bromden McMurphy is).

Ghengis Khan was a conqueror, like Napoleon, who covered much of the world that he could "get to." When McMurphy took the guys out, he was showing them a world about which he knew more, but he was not from the area where he goes, so he, too, is exploring.

Attila the Hun was a barbarian and Nurse Ratched certainbly brings out the violence in him as he tries to rile up the other inmates. He, until his frontal lobotomy, cares little what damage he leaves in his wake as long as he causes major trouble.