To recruit Langeais, you need to complete the mission "A Wave of Ghosts".
Mission requirements: Destroy the Espiloia Kinghts.
Recruitment requirements: Langeais does not retreat.
Once you have unlocked the mission, simply complete it without Langeais retreating. She will be present from the beginning and will be on a raised area in the center of the map barracaded by boxes. She will not move from that spot and the access to it is very narrow, so just block the access and let the enemies come to you and you should have no difficulty.
The achievement will not unlock until you save your game.