As Schaafy said, this is missable. And you need to do this on Refrain mode.
This achievement actually need you to date both Excella and Jinas untill you can take them into the final battle to get their own seperate ending and
they both give you their own personal sigil that you can use in the grimoire, DON'T DO THAT.
keep those sigils on you at all times untill you got both their endings and went to talk with Alfriede. Talking to Alfriede should unlock the achievement.
I'll post this video here that explains it perfectly. (Not mine):