Go for the United Front option in the Xian incident, and move all troops towards the northern coastline plus Shanxi. Assuming you move your forces quick enough, you will arrive in time for the Japanese invasion. Hold your position in roughly the middle of the Shanxi land, and keep the IJN stuck in a couple coastline cities. Send troops from your allies to help. Once you have enough troops and the IJN forces in Shanghai (and the area around, if they spread) are low on supplies, counter attack. Eliminating them should hurt Japan significantly, and you can then keep a small garrison at Shanghai, and use the rest to push into Manchuria. Encircle when possible, and abuse your allies's armies (In particular, use the Communist Chinese troops for the most important and/or hard battles, since they have very strong troops with good doctrines). Eventually you should be making progress. Annex (Or puppet, if you prefer) Manchuria, and conquer Korea.
From there, it would be wise to set up a navy. Don't attack until your navy is strong. Let the Japanese launch Pearl Harbor and invade the Pacific, and, due to the loss of land, get crushed by the Allied forces. From there, use your navy to land in Japan and smash them before the other nations arrive.