KINGDOM HEARTS HD 2.8 Final Chapter Prologue

This Achievement is very easy to do, just happens later in the story. You will unlock all the flashbacks in the third set of worlds after playing through as both characters. If you go with the suggested level, it will happen during the Symphony of Sorcery World. If you don't view a flashback when you unlock it, you can do it from the pause menu under the momentos section. So it's not exactly story related, but you will have the means to do so from playing the story