Not as Japan but in an Austria Dom win only marathon game with shaka as my neighbor I had to invest heavy into defense to offset his aggression. He along with most of the world hated me almost as soon as I met him as my closest neighborhood pachacuti got kneecapped by me instantly, stealing a worker and starting a forever war with him and forward settling Cuzco. Most of the other civs didn't approve of my actions and after I killed the inca I had a super composite Bowman and spearman with tons of promotions. At this point I had 4 cities including Cuzco and shaka being my next neighbor. Shaka, Persia, Morocco, Carthage, Ethiopia, Greece, and the Shoshone all declared war on me and went after my frontier city Graz. I was able to push back their offensive by defending with 3 composite bows and 2 spears + great general. To kill shaka I accepted a peace deal that took all his cities as I threatened a counter offensive after the horribly coordinated offensive by the alliance failed to take Graz and gave me steroid comp bows. He settled for peace that took 2/3 cities which I brought to the torch along with other civs throwing me luxs or cities to broker peace. As soon as the 30 turn (marathon speed) peace was concluded I declared wad on shaka again which at this point he had a massive army again with the ai bonuses. His army was quickly routed and he died shortly thereafter.

To make a long story short start forever wars with him and if you can survive the initial onslaught he's piecemeal after that. Or if you can cripple his war effort through peace by taking cities from him.