The Land of Frozen Fortunes refers to levels seventeen through twenty. This achievement will unlock when you kill the enemy general on level twenty, End of the Line, 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.
End of the Line is the last level of the campaign but the same basic strategy applies. Get your archers to demon status ASAP and capture the four shrines near you as soon as you clear out the closest enemies. Salamander monks work the best, and you will want to focus on getting your general and both monks to demon status once your archers are up to par.
All of your enemies start as demons, so it will take multiple units ganging together to get kills, especially at the beginning. You have quite a few more units than you need, so if you lose a few at the beginning it is okay, but try to not lose any archers. Also, you will want 3-400 rice for the end game. Defend the top of the stairs at the beginning, and then spread out to defend the two bridges. You will want to send a cavalry unit to capture the two central cleansing shrines early on.
Play defensively, and pull back wounded units to heal on the shrines. Any unit with a blue sword will freeze you, but good use of your archers should prevent anybody from getting into melee range after the first few enemies. Remember to protect your archers by having them fire from behind defenders whenever possible.
When the enemy general finally wakes up and attacks you, the fastest way to kill him if you have two Demon Salamander Monks is to put them behind your general, and no units out front. You can't kill the enemy general with units other than your general, but you can damage him with other units, cleansing shrines and spells. His attacks will freeze you, so ensure that you are attacking first. Use your two Demon Salamander Monks to first cast three lightning storms on the enemy general. Save your game and load it up on all of your devices to synch the achievement, and then attack with your Demon General. Game over!
*** 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 twenty 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!