This achievement unlocks after speaking to Zeliska outside the item shop when their affinity rank is at least 4.
UPDATE 1: Thanks to STOUTASSASSIN67 for pointing out I hadn't mentioned when you get her. Now updated.
In order to get Zeliska in your party you need to advance the story and along the way your make friends with her, and be able to have her join your party.
You can check your current rank with any of the character by pushing cn_start then go across to the player icon and click on friend list, you can then push cn_X and down the bottom on the right, it will say their rank.
The fastest ways to gain points with the people in party is by reviving them.
On the Extreme difficulty, the snipers one shot everyone. Using the Photon swords skill Sword Barrier you can swoop in and revive them.
On the Normal difficulty the way I've been grinding this out is by fighting the Honeycombed enemy in Solitary Sand (Large mountain to the right of the map). This will require luck and being good at dodging.
The alternative solution is take on quests and go farming monsters. This will also help to level up your characters.