Having gotten Romance of the Three Kingdoms myself, I'll tell you the easiest way I've found to get it:
Start a new game as Nationalist China. Immediately click on Japan and see what NF they are doing. If they are doing "Purge the Kodaha Faction", quit and start a new game. Keep doing this until they pick "Support the Kodaha Faction" (I've never seen them do "Unthinkable Option" or "Strengthen Civillian Government" while playing as China, but if they pick those two you can keep playing). By going "Support Kodoha..." Japan will not focus on war with China (same for Communist and Democrat paths) which gives you all the time you need to unify all of China, so you can declare war on Japan when you are ready.
BUT!
You must declare war on Communist China and annex them as soon as you can. They get a NF that lets them blow up factories in Japanese territory and frame you for it, which causes Japan to declare war on you. Smack down Communist China fast enough and that won't be an issue.
Do your focus to demand submission from the warlords, and go from there. If Yunan and Guangxi refuse to submit, declare war on them and puppet them one at a time. As long as you annex them through the puppet autonomy mechanic, you will get the achievement. Just don't annex them through war! Once China is unified and you have built up your army, do the NF that demands Japan submit to you. If they refuse, go to war and annex them. Watch out for any factions they may have joined (neutral Japan still often ends up in the Axis due to starting a war with the Soviets) because you may find yourself having to capitulate Germany and Italy to win the war with Japan...