The Land of Monsoon Meadows refers to levels nine through twelve. This achievement will unlock when you kill the enemy general on level twelve, Forsaken Flights, 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 twelve: Forsaken Flights features Oni, similar in appearance to trolls. They will attack you or the enemy if either of you get close to them, with great power. Goad the enemy into walking by the Oni, thus letting the Oni do your work for you. There are skulls scattered near the entrances of the Oni's caves. Leave them and the enemy will try to grab the skulls, upon which they will be attacked by the Oni. When the enemy numbers have dwindled, avoid the Oni as best as you can on your ascent, and take out their General. Whether by traditional means (attacking him until he has no hit points left) or by forcing him off a ledge by hitting him when he is close to it.
*** 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 twelve 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!