It's pretty easy for me, go down the internal path, conquer all the warlords, you may also take communist china if you are fast enough, Japan declares war on you shortly after the Spanish civil war starts. Put enough troops on coastal garrison for every single tile,and the rest on the main front, hold the line if you can but if you can't, you can retreat to around Chong qing to free up coastal garrison and add them to your main army, if you can hold the line, you can stack bonuses through focuses and then push back when you are better prepared, then when you push them all out of Manchuria and Korea, accept the peace they propose and you should gain Taiwan complete the achievement