The Land of Eternal Blossoms refers to levels five through eight. This achievement will unlock when you kill the enemy general on level eight, Guardian Gardens, 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 eight: Guardian Gardens is a medium sized map. It features the first appearance of Demon Monks -- healing monks who are allowed two turns -- which means in terms of strategy, you need to be aggressive with your attacks. If you're going to attack, make sure you are able to kill that particular unit in the same turn. Otherwise, the monks will simply heal it back to full health, essentially resulting in a wasted turn for you.
*** 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 eight 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!