1; the first thing is you need to be prepared to grab as much ground as possible before the debuff kicks in. Have as many special forces as you can and try to improve your fighting capability as much as possible. If I remember correctly China needs army experience to remove their starting debuffs so its actually prudent to put off the war for a while.

2: try to beat the Soviets in the border dispute before fighting China; you get some useful research bonuses.

3; Once the war starts you need to try your best to grab hold of at least one of the cities in the timer to put off the stagnation. With that in mind it makes sense to go after them one at a time and plan ahead so you don't waste time moving your armor around.

4. You have superior industry; put it to use and always use CAS and fighters to put the odds in your favour.

5. You can use the Spanish Civil war as an easy farm for army XP and to level up generals, depending on how long you want to delay invading China for you can keep switching units/generals to gain a meaningful edge.

6. Once the war starts and you've done at least one big encirclement, pull all your marines out and take every port you can; not only will this cut off most of China's import routes it will also give you several easy vectors of attack.

7. Once you come to Changdong and Wuhan you have a tremendous opportunity to pull off what should be the final big encirclement you need; Chinese armies tend to lack real piercing attack so one or two moderately up to date tank units will be devastating; there is a road between Wuhan and Changdong and if you reach either city but don't push too hard in between them you can sail down with an armored spearhead and cut the supply off for at least one full army; if you complete the operation successfully by that stage the chinese frontline should be too thin to stop you taking them out.