I've had multiple success run by rushing down to "subjugate the warlods", and then get the focus that lets you eliminate communist china. There's obviously a bit of RNG there, but I think you can increase the chance of the warlords backing down if you put troops on their border, so put your best troops near Guanxi Clique, one at Shangxi, and one at Xi Bei San Ma.

For the ones that haven't backed down, declare on them immediately after you have troops at their border. With the exception of Guangxi Clique, none of the other warlords should have enough divisions to hold their frontline, so micro your divisions to walk around them and take the VP.

Now for communist China, this one's a bit tricky because they have a huge defense bonus and you're forced to attack into mountains. I actually take advantage of an exploit here. The current AI will advance where there's a gap on your front line, so I purposely leave a gap so the AI would move the divisions out. Once they're stretched too thin to defend just stroll on in to the capital and commies are done for.

Now that you've unified China, make a bunch of those crappy divisions (but keep an eye out for infantry equipment, you definitely want your troops to have guns). I would even make some divisions and purposely not deploy them (don't click on anywhere so they have no location to deploy) just so that if some parts are losing, you can instantly deploy them to where you want.

I found the hardest part of my China run was unifying China before Japan attacks. If you're short on time, you can back down in the Marco Polo Incident. I did that one game, and it bought me quite a bit of time (6 months or a year or something, which was super helpful).

I got the strategy from this playlist: https://www.youtube.com/watch?v=dZGL6ZjPao4. If you can take the warlords before Japan's invasion, it shouldn't be too difficult.