From the streams I'v seen so far, Twintania doesn't seem bad at all, when you beat her and Nael drops down is when the fight immediately ramps up to borderline crazy, unless you're already familiar with the final phase of T9 from original progression, then it's more nostalgic than anything, and you just have to figure out new dance steps with your current members. I haven't seen any of the combat with Bahamut yet though.