Ooh - how about Zenos as EVERY BOSS OF 6.0!
81 Dungeon Boss: Zenos
83 Dungeon Boss: Zenos
84 Trial: Zenos sitting on his throne
85 Dungeon Boss: The dungeon has nothing to do with Zenos, but when we reach the boss, Zenos descends from the sky Sepihroth-style, slays the boss, then we fight Zenos!
87 Dungeon Boss: Zenos and Fandaniel tag-team
89 Dungeon Boss: The whole dungeon is a mystical
Journey to the Center of the Mind inside Zenos's head.
89 Trial: Zenos possessing Zodiark
The level 90 dungeons are optional, but somehow also involve Zenos, in spite of the fact that we wiped him out along with Zodiark.
SE Devs: So, we heard you all can't get enough of Zenos, so here you go - all Zenos, all the time! Our best villain EVER!