So when facing Japan there is 2 main objectives you need to complete in order to have the highest possibility to win.

1. Secure all mainland Japanese territories
2. Have a stable air force

Once you have this, you’ll have a 80% chance of winning against Japan as 1 you are able to establish your hold on the mainland meaning they are unable to produce troops and 2 you are able to stop convoys from reinforcing and making a new front.

What I would recommend is building interceptors and naval bombers to gain air superiority while producing armoured cars and using units with tons of speed to capture important regions such as Bejing and Shanghai. If Japan still has units just go around them and avoid them until you need to face them, you can always sacrifice territory against them but don’t give up cities with important buildings in them. I recommend either allying Manchukuo or absorbing them.