I find the Sentinals to be paramount to soaking up damage when a boss telegraphs a large attack. The problem is getting to the paradigm in time to make it work. I usually keep at "Sentinal Sentinal Medic" paradigm at the bottom of the list, so I just press up until I reach it. Also, the Sentinal route on the Crystalium gives you the biggest defense boost and HP. It's worth it to put at least a certain percentage of your points into Sentinal just to get the attribute boosts.
In response to the "blank card character" comment. I think there's possitives and negatives to both. The disadvantage of completely individualized characters is that, unless the game system is PERFECTLY crafted (and I mean perfectly), some characters will always be more favorable than others, and eventually, you wont use them at all. At least blank card character systems let you craft characters that you actually want to use. And also, I totally disagree about FFX... I think that's a top example of individualized characters. Even though the sphere grid ultimately opens up to everyone, for the first 2/3rds of the game, every character is set along a path that really emphasizes certain attributes and abilities. Yuna only does a quarter of the damage of Auron, Lulu is the only one who can cast magic, etc. The characters become VERY focused in that game, probably more than any other game in the series besides FF9 and FF4. But you're correct about FF6, 7, 8, 12, and 13 (especially 12).
But I don't see how this applies to FFXIII-2, there are only 2 characters, and they are VERY focused, at least towards the beginning. Noel has much stronger attack, Serah has much stronger black magic and healing. Though I mostly let the monsters heal.
On other things. I'm guessing there are alternate realities that allow you to get certain places in an area that aren't available in others? There's a mesa in Yaschas Massif (10AF and 1XAF) that I can't reach, with a treasure. Even worse, there's a box treasure around the populated area that's hidden behind a bunch of creates. Also, Mog reacts to something there, but for the first time, innitiating a temporal search doesn't yield anything (??). I've tried EVERYTHING from jumping off ledges with a chocobo, finding alternate routes... I just can't get to that box.
PS: I am finding that I have to grind a bit, just for some thngs. The first Chaius battle was a BITCH, he kept killing my healer, I had to use up lots of Pheonix Downs. I grinded a while and was able to bearilly beat him. The game isn't super hard, but it definitely doesn't play itself.