You can skip all the dialogue by pressing X, EXCEPT WHEN TALKING TO MAECHEN. When he talks to you at Youth League HQ in chapter one, you have yo just sit there and listen without pressing a thing, and that includes even presing X to say "go on." Also just leave the controller alone when he talks to you in Guadosalam in Chp 3. and then on the airship in Chp 4.

You can safely skip all other dialogue (not scenes with square button) and still get 100%