After picking the 3 characters you're going to take to the final boss fight with the Archdemon during the Denerim climax, you'll be faced with a number of battles through the Market District, the Alienage and the Royal Palace before arriving at Fort Drakon.
During these battles you have to ability to summon armies within the 'Advanced' section of the Radial Menu. To unlock this achievement, you must keep half of these armies alive and it unlocks upon arriving at Fort Drakon if I remember correctly. During the Archdemon fight you can use the armies as much as you want.
The most certain way to get this is to just not call out any of the armies. It's not even necessary on the easier difficulties anyway.