Well, I attempted Seph again. Level-wise, I think I should be good. I don't want to grind to 90 for second chance, although I might grind until 75 for the second mp rage.
I managed to get him to his second phase on my second attempt and died due to using strike raid too slow when he started his 1hp/0mp move. Third attempt, I got him pretty far down his second phase and he omnislashed me. I forgot how I survived that one back then. I think I just guard spammed. Maybe I could abuse leaf veil in this one, but it might cost too much mp to keep it up for the whole duration.
This is still my favorite optional fight in the series (maybe until I fight the new one later or when KH2FM comes out).