The Shores of the Dead are the first four levels. This achievement will unlock when you kill the enemy general on level four, Glorious Gates, 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 four: What you want to do is attack the enemy unit directly in front of their General, then, when your next turn comes around attack the General with everything you've got. In the bamboo directly to the right of where your units spawn, there is a potion. It can either be red (bonus attack strength) or green (bonus health and regeneration). In either case, you should pick it up.
*** 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 four 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!