Complete all maps to unlock the Einhenjar feature. This is an extra bonus that is used automatically while in battle. You can either spend coins to get the map pieces faster (at the "Research" tab) or by playing the map levels repeadly. You can deconstruct existing Einherjars to get materials required to upgrade the ones you have. You also have a daily chance to randomly improve their skill by tapping the "Update" button at the menu.