as far as I know, most (perhaps all) Japanese graduate schools do not use GPA for admission. They all uses examination tests.
So, the first thing to do, I think, you should go to any school or program websites you want and see their past entrance exams to see if it's too difficult or not. You even don't graduate. I think there is enough time to prepare (gather information about your interested program, what the range of topics they are tested?, how difficult are they?,...).
The universities in Japan I've often heard are Tokyo,Kyoto,Osaka and Tokyo Institutes of Technology but I think you need to look further at their AI program about their curriculum or how interesting it is, for example.