To get this achievement it is unknown if it unlocks just by beating the game or by taking certain choices, the choices I took in order to gain the achievement are as follows.
-I'm pretty damn curious
-Go in with them
-Well, she did come all this way. I'll run off with Myra before Rin notices I'm gone.
-Tell him to genuflect.
-Confess
-Go to the school rooftop
-Stand up
-Go to the school rooftop
-Stand up
-Go to the school rooftop
-Stand up
-Go to the hill
-Myra's Diary (On the main menu after completing the game)
Tips - I recommend to speed things up to go to the settings and turn your text / auto speed up full and under the skip option click all this will allow you to go through most of the game almost instantly to get to the achievement spot faster.
List of options credit goes to GameTHAT on youtube.