AI prioritizes debuff or interrupt mysteries first. in order for your first slot ninja to go first it would have to be a lower cost skill or of higher debuff quality. Positioning and placement order only matters when it comes to which ninja gets shielded or who gets a buffing mystery( I.e. Bee's myst). Aside from that you have to build well rounded comps in order for your AI to function properly. Best example is my Earth main Onoki team. AI prioritizes onoki's myst first simply because it deletes people from the field which would be considered a higher debuff status the my earth mains hulk smash mystery.