Welcome to Gaia! ::

I realized how to get Luke to do TWO mystic artes: his normal one and then a special one called Ion right after it. Here is how:

NOTE! Ion MUST be alive and has not gotten kidnapped before attempting to do this.
(A handy thing I did was beat the game then spend some GRADE on the things that let you have all AD skills and all artes)

1) Get into a battle while using Luke as your character.

2) Activate Luke's overlimit.

3) THIS IS IMPORTANT! Make Luke use an arcane arte that has the element of LIGHT(ex: Light spear cannon or Guardian Field) while holding the X button to preform the mystic arte. Luke will preform his mystic arte like normal, but right after he's done, Ion will appear and use a Daathic fonic arte.

Oh, well i think a mystic arte within a mystic arte sounds cooler