My work around for getting a decent game against China as Japan is the following: 1) I edit all infantry/militia units of China--Nationalist, Communist, et al--to model 0 or 1. 2) I edit Chinese infantry tech to a max of 1020--which is 1897 infantry tech. 3) I give Japan 1939 infantry tech at the start. 4) I edit about half of Japan's infantry up to 1939 infantry at the start.
Doing these things still allows China a crazy numerical advantage but with Japan's 1939 infantry going against Chinamen with muskets and pitchforks, Japan is able to win many early battles quickly. Even so, once you push a certain distance into China and the front start to widened, it still is a challenge to make progress when every province you face has 25+ entrenched divisions--even if these division are 1897, 1907 type stone throwing riffraff.
I seems to me that the Chinese army of 1936 was in no way technologically in the same league as Japan's. It seems they must've been several generations behind. My fix results in a fun--and no where near easy--game against formidable China. It is a race to push quickly through so that China does not have time to research higher infantry techs. For if China even comes close to Japan in infantry tech, Japan is toast.
It's easy to make these mods if anyone wishes to try. Just make sure you copy any INC files into a backup folder before you edit them. One , or { out of place hoses up the works.