[Arryn]

Your fleeing mages walk. Tritons fly. Tritons being much faster, reach you before you escape and thus kill you. End of sad story.

EDIT: some advice: protect your mages with troops having orders of "Bodyguard". It may not protect you completely against enemy flying troops, but it's certainly better than them being all alone and highly exposed. BTW, having mages alone in the back is an invitation to enemy archers & mages with orders of "fire rear".