When I play as Japan, I entirely ignore China.
Aside from taking an empty territory here or there. The only target worth shaking a stick at in China, is destroying the “flying tiger” fighter on Round 1.
Manchuria and kiangsu can hurt if chinese forces take them from you (allowing UK bonus) but without the fighter China has a difficult time taking anything. and even if chinese forces made it to the coast, it’s very easy to obliterate them.
Remember that taking 1 chinese territory only gives you a +1, vs, if you took russian or african territory, you get +1 and the allies get -1. So it’s twice the money to go anywhere else.
China could have 1000 chinese units on it. But it wouldn’t matter, because they can never attack outside china. It’s best to focus your efforts elsewhere.
IF however, you are dead set on taking out china, the only effective campaigns I’ve seen usually can involve a complex in Manchuria, or 4-6 tanks combined with fighters and the odd inf, in a quick blitz around turns 3-4. The general rule is either entirely commit, or don’t at all.