Agin's approach
If you play Nat. China, start with producing nothing but Infantry divisions. Try using as many "good" officers (i.e. Offensive, Defensive and Tricksters) in the beginning war against Communist china as possible, the experience they gather in this war is highly valuable against the Japanese later. As soon as you make your peace with the Reds, hurry your troops down south to engage Guangxi, which will attack you roughly three months later. Here applies the same rule: use as many "good" officers as possible to gain experience. When you make peace with them, hurry your troops north again. If Japan attacks, forge the "Unified Front" and seize command of all the troops of your new allies. Send these troops to the provinces with beaches to prevent the Japanese from invading. Look frequently if they have produced new units and transfer them to the beaches, too, to strengthen your defenses.
From time to time, share blueprints with your allies, like better Infantry Divisions or Land doctrins.
Send your troops to Guanxi to slow down the inevitable japanese advance, but don't save Guanxi, even if you could - it's existence prevents you from conquering all the jucy Japanese provinces and the whole of Manchukuo later!
Don't get cocky, even if it seems tempting to attack them or take the occasional province back, don't!
Wait.
Let them attack you, your defence is usually much better than their attack. Watch the terrain on which you fight closely Mountains and cities are your friends! Let them bleed themselves on your increasingly higher stacks of troops and contend yourself in gathering more divisions.
When you have built about 100-120 divisions (around early 1939), you can begin to advance. Make sure you have enough Field Marshals to support your huge armies when you attack. Before you do, check the weather. Then attack them a while and stop to have your org back and your casualties replaced. You should get them back much faster than the Japanese, and your manpower pool and manpower-regain-rate is much higher. Thus, you simply wear them out until they break. Then you hit them with the entire brutal force of your Army. Push them further on and into the sea. You should hurry a little, because all of this has to be acomplished before Vichy-France is established! If Vichy comes, the Japanese will get Vietnam and could start invading your country from the south. So it is crucial to send some 50 + divisions to the border of Vietnam to be able to start invading it as soon as the Japanese get their Hands on it. As a bonus, it has some valuable ressources.
If you have done this, you can finally relax a little. Build some tanks, build some brigades for your troops (I always use Artillery). If you want a really big Empire, move your troops to Afghanistan, DoW them, conquer them and then invade the Soviets! The trick in doing so is to drive a wedge from south to north, thus cutting the USSR in half, starving the eastern soviet troops until you crush them and advance westward towards the Germans. Remember to advance to Baku quickly before the Germans take it ^^
If you encounter Ressource-Problems, enhance the Infrastructure of ressource-rich provinces (they go up to 200% now and the amount of ressources they produce increases greatly, as well as the number of IC the factories provide). This is cheaper and faster than building factories.
Okay, that's it. Let me know if it worked for you!