I'll give you my strat to easily defeat China with Japan (in SP historical with expert AI)
You want to start the war asap, so you'll not face united China and can pick chinese nations one by one.
pre war build up :
Focus, just rush marcopolo incident. While doing so you will build civ industry in your province with high infrastructure so you don't lose very much not going to war economy.
With Japan, you just have 3 oil. Your industry is very weak so you don't want to import anything. The good point is that your oil reserves are full. That's enough to defeat China. Save your oil (just train your fighters) and don't build anything like new planes, tanks, mot, that use fuel. So your MIC should be use to build infantry & support equipment, and artillery.
PP : save 150+ for silent workhorse, then one army decision (that one give you 4 new military factories)
require from puppets all their divisions. Train all your army (one of your garrison is green, disband it). Switch all motorized/tank/marines to infantry template, saving back your trucks and light tanks. Those will be very helpful later.
You should aim for 72 division (3 armies of 24), you'll not have enough equipment to raise more.
pre war deployment.
You'll initially attack Shanxi alone. Deploy 7 infantry divisions in Front of Beijing. 4 more will be use to naval invade from Philippina the second southest city of nationalist China (that one is surrounded with mountains). Every other infantry divisions (10 at least) and your cavalry should be deploy facing Shanxi, one group ready to rush it's capital through forest, another to go as east as possible through desert. Bolster your naval invasion with 6 more garrison divisions, fill any gap on the border with garrison but be very careful to not exceed supply limit. Finally, use your TAC bomber to get air superiority over Shanxi.
War with Shanxi :
When Marco polo bridge focus is completed, declare war to Shanxi. with pp, escalate war in China as often as possible, and improve workers condition asap. You should meet very little resistance but you'll have to occup all the country before they surrender. When they give up, strat redeploy everything in South Shanxi (just left a frontline filled with a few garrison ofc along shanxi border) and drawn orders to breakthrough then march along the river until your reach the coast. Doing so will allow you to encircle half of the Chinese army.
As soon as you have xp you can tweak your infantry template with support artillery. Then duplicate it and replace reco company by a tank reco company : that will give you very little armor, but that's enough against China WW1 equipment to get the armor bonus (which make them 4 times more effective). You have enough light tanks for 15 divisions (I recommand to spare a few tanks, so 14 divisions). Be very careful to never use those divisions in moutains or mud, because you have very few spare tanks.
War with nationalist China :
Start war against nationalist China. Attack Beijing, now little defended by overstretched Chinese army while your main force breakthrough from southern Shanxi.
Your naval invasion in south just aim to attract some troops, especially soviet volunteers. You should quickly assaut the port (with naval fire support, don't forget that), then just advance a bit on the mountain around and stay.
After pocketing such a big part of the Chinese army, it should be very easy to make more encirclements.
Ending the war, annexing/puppeting whole China :
If you're successful, you'll need to stop yourself, waiting for China to complete the united front focus. If you don't, you'll get the peace conference just with nationalist China and Shanxi. When China has created united front, it will drag every other Chinese nations in war. You must get at least a frontline with each of them so you can annex/puppet them during the peace conference. Once you're sure to get them, finish nationalist China.
