Spectral Force 3

To recruit Margaret, you need to complete the mission "Hunt Elves".

Mission requirements: Attain High Mercenary rank and the Elf Alliance is destroyed.

Recruitment requirements: Complete the mission. Defeat Margaret and then, when given a choice, choose "Persuade her to makke her join the Norius mercenaries".

Once you have unlocked the mission, simply complete it by defeating all units, including Margaret. After the battle, you will be given a dialogue choice, so choose to persuade Margaret to join your group. All the enemies except Margaret are relatively weak, so you should have no trouble with this one.

The achievement will unlock after you have saved your game.

Thanks to LV 1 Blue Slime for this pertinent tip:
Although it isn't particularly relevant for most people, I thought I'd go ahead and put this right here for people who are part of the stackables master race : at the end of this battle you need to choose the second dialog option, not the first. Choosing the first means you won't get her to join your team and won't get the achievement.