Shulk
Stages 1 -6: Keep the same
Boss: Egil - Prism Tower(stage) - Obstacle In Our Path(music)
Egil is outside of Yaldaboath. Sword items appear, Egil will use them and jump around, dodge, and throw laser projectiles. He will also throw swords at you if another one appears that he wants and use the new one, and if it's plausable for bosses to do such things, he will disable Monado arts temporarily when he's at low health.
Too many bosses are big slow things, so I took the mecha away from him and had him outside it for a epic badass sword fight.