The Land of Golden Harvest refers to levels thirteen through sixteen. This achievement will unlock when you kill the enemy general on level sixteen, Autumn Assault, on Normal. If you at any time up to this point have changed the difficulty level, the achievement will not unlock! Save before attacking the general and open that save on all of your devices. Kill him near simultaneously on all devices and you will get this achievement on all of them.
Thanks to Shelton for tips on level sixteen: Autumn Assault is a large map, home to many cleansing shrines, as well as crow monk shrines. Camp at the first pair of cleansing shrines, claiming them, and then kill the enemies as they come to you. Be careful of the archers to your left. I also suggest awakening your General early in proceedings and transforming him into a demon. He will be paramount to your defense, just remember to pull him out of the action after each turn. Once you've dealt with the enemies spawning in your immediate area, advance up the steps on the top right of the map. Claim the two cleansing shrines there, then make another defensive stand. When you feel comfortable, attack the General.
*** Note that this achievement can become permanently unobtainable on some devices if you unlock it on one without doing the simultaneous kill on the others. For instance, if you play on the WP and unlock this achievement, and then later complete level sixteen on the Xbox, for many people the achievement will not unlock on the Xbox even if playing offline on a clean install. This glitch applies to all of the campaign achievements!