Tried the fight again today. I said 'fuck it' and stopped trying to do this one Aeroga strat that I found on youtube, and just played like I normally do.
Got him on my first try lmao.
I equiped the accessory that gives the character their max limit break. As soon as Darkstar appeared, I used Finishing Touch on it, which removed over half of his health. Then I wailed on it with Prime Mode, summoned Odin as soon as I could, and Odin ended up finishing Darkstar shortly after. When Odin did Zentetsuken, Rufus fell. lol.
Ayyyyy nice.
My big mistake when I was doing that fight initially was when I was close to finishing, but accidentally summoned, and then suddenly had no ATB, went "oh fuck" and Rufus cornered me and it was over lol.
I have trouble with Rufus, I'm not "good" enough at the battle system, but I felt very proud of myself for not bumping it down to Easy and getting it done after a couple tries haha.