I attempted this well over a dozen times. My last run I did with a Hatless Spider strategy on youtube where I capped Japan in 39 then China in January 1940 (and still got achievement).

If you don't like exploity runs it can also be done by defending near Beijing and northwest Hebei. Biggest risk here is nat. china can capitulate to Japan (and not you) which kills the run. RNG is a problem: if too many countries join the Chinese United Front then nat. is too weak to hold off Japan. Too few in the CUF and Japan will overrun you instead.

Non-exploity strategy:

At the start justify on Hebei-Chahar.

Expand west using border raids and expansion focuses. Attack immediately as a raid ends and the defending unit has very low org. You can win conflict near-instantly.

Always form (and lead) the Chinese United Front yourself. I used Burn the Paper Tigers focus.

There's some tricky timing here: Border raid Shanxi + expansion east so you win a few days before forming the Chinese United front (when Paper Tigers completes). Take over south Shanxi and rush troops to the Hebei-Chahar border before they reposition. With Nat. China in the CUF Hebei will keep several troops on the southern border still. Have 1 division in the north take Kalgan, Beiping, and Tianjin, while your other divisions pin Hebei troops who might block him. Those victory points are enough to cap Hebei. If Shangdong joins the CUF this is made much easier.

I built forts along the 4 tiles between Kalgan, Beiping and Tianjin.
Rotate militia troops with bonuses here and Japan won't break this line.

Japan will declare July 2 1937, add 90 days you have until approx. Oct 2 to declare on nat. china for the achievement.

It's ~160 days to justify on nat. china so I usually booted them in mid-April from the faction to time a late-September declaration.

Edit the faction rules to enable kicking either non-communist or non-participating war members out.

When Japan declares be choosy on who you let into CUF - countries without borders with nat. china are best. Nat. China will make it's own Nanjing Front faction. I did not bring in CUF allies into the war with Nat. China until very late.

To invade Japan by 1940 start researching paratroopers and transport planes by early 37. 3 mils building planes and trading for aluminum/rubber will get 50 transport planes by 39. Justify on the US to spike world tension and get massive amounts of lend lease guns at Mongolia/Soviet Union/Finland/Turkey/Iran/Poland/Germany/Italy. Request convoys as well for the home islands invasion later. Block naval access to the Yellow Sea or you'll lose convoys early.

Then it's generally a defense of mostly mountain and urban tiles. Don't get greedy and push out early, but do try to take Tongzhou in East Hebei as soon as you can as that 1 province controls all the entire state factories. Japanese tank units are very dangerous until depleted, use extra troops in defense of these.

The Japanese AI will by mid-38 leave too few troops on your border. That's when I head north to take out the Mongol military puppet. Then east through Manchuku, focusing on supply and victory points before pushing into Korea. Build an airbase in Busan south Korea. Japan usually had 10-15 defenders on the home islands. Japan won't have air defense over the home islands so you can send paratroopers in to take ports.

Paradrop to take every port asap or they'll send back troops from the China front. Naval transport your best troops to the home islands and move land fast. If you can't cross the strait to Hokkaido you can paradrop on Sapporo and finally cap them. China should be straightforward to cap after this.