Garuda Ex, as the fight goes on, the mechanics DRASTICALLY change and how you deal with it (last 40%?) is completely different from the beginning.
Levi Ex, and Shiva Ex, are really easy fights. Levi - don't fall off the edge, make sure you charge the thinger enough and set it off at the right time. Shiva - don't slide into he wall, and if your a tank, you have a bit of mechanics to watch oiut for to either force cleaves or avoid cleaving the party. Otherwise, very BORING fights. Also Garuda Ex makes me facepalm the most, as I can't believe how many times I've been in DFs/PUGs with people who jus can't get the mechanics and we wipe.
T2 - I don't feel like anything you said bears any relevance to T2? are you saing fighting the nodes, VS fighting ADS should be the same? ADS hands out it's worst mechanic almost right away. It only gets more difficul because you get more Allagan Rot to deal with. . . but then . . . most people don't even do Rot anymore.
T6, ehhh, you can beat that fight in like 3 and a half minutes or something, so i don't see how it's a good example of fighting something for 10 minutes to then learn an example at the end of a fight. If it's taking you a long time to get there, it's because you still don't know the first mechanics. IMO, it actually gets easier at the end.
horrible Design: OK, I disagree with you, but I understand what you're saying about T9. The dance you need to do with the elements I always thought was really wierd. I have no idea how SE thought of it, or originally planned for it to be beaten. but let's face it, that's almost your only mechanic to deal with at that point. and topping your tank off/using CD's for bahamut's Favour. For DPS it's easy street.
but T5?! T5 is actually an amazing fight, and very well done. I think it slowly ramps up in difficulty, the hardest part is probably actually dive bombs + snakes, which really isn't that hard. Then it slopes to gradually getting easier.
After Aetheric profusion, what kills people, is Twisters and Dreadknights. People who aren't paying attention to when Twisers is being cast, and it kills them, or people who don't know what to do for dreadknighs. But the knights really aren't hard to kill anymore. You just have to say away fom the center, until you know who is getting jailed, then all the DPS need to switch to the new target right away, kill it, go back to Twintannia, but it's not DIFFICULT. If you employ a decent stratefy, i gets significantly easier. and then the last phase, is serious fluffcakes.
I think I've only ever wiped on it once, and that's because we had no idea what we were doing when we got there.