SquareEnix just isn't what it once was, for it took them 5 years and 2 games to make a new battle system.
When you get down to the real nuts and bolts of it, the battle system for Final Fantasy XIII-2 works, and doesn't work badly at all in my opinion. What makes it difficult to understand is the interface, and the oddity that the visual User Interface doesn't reflect the battle system in place -- it merely reflects what appears to be something that is classically Final Fantasy for screenshots alone, but doesn't handle that way.
Actually, you may find yourself completely ignoring the User Interface entirely, because most of it is useless information or visual nonsense that can distract you as a player. All that really matters is HP and whether or not you're buffed or debuffed, all else is needless. The only time you'll be paying attention is when you do a Paradigm Shift, for this is where the battle system hides itself.
Attacks, Spells, Skills, Abilities, all of these things can be and are often done automatically for you. So instead of actually having control over the scope of the battle, you just have control over the types of abilities your AI has access to and when. Long gone are the days of carefully planning out attacks or finding out what the weakness is your target the hard way, most of this leg work is done for you.
Combat happens fast, and if you've programed your AI's flavor in the right way, you can pretty much end battles in a handful of seconds -- sparing the minute or two you may spend on an actual challenge. For instance, you'll find yourself facing against a large crowd of critters. Well if you Paradigm Shift to Ravager (Mage) and had Wide Range activated, your party will cast AoE spells that destroy everything. If you keep using that same Paradigm Shift, the AI will use the best spells for the time and they'll even switch between AoE and Single-Target all on their own. So there is no reason NOT to pick Wide-Range for Ravager.
With most of the difficult stuff down for you, all you have to do is focus on switching between Attack, Magic, Buff, DeBuff, Heal, or Defend. All of the little details like What-Type-of-Attack and What-Type-of-Buff has been removed from the player's equation, all you do is pick one of the SIX things you can do, just in a different order arranged between THREE characters.
Of course you can customize creatures you capture to fill out those roles better, but ultimately you are still just telling the computer what to do from the last of six options. Though you are given two main characters in the cast, they really have no real customization to them. Sure, you can level them up in choice classes, but as you quickly draw towards the end of the game they'll quickly fly through levels you'll easily have several mastered classes.
Annoyingly enough, Serah and Noel, the two primary characters in your party who never leave it -- differ greatly in ability. Serah is given far weaker overall abilities, and is pre-determined to specialize in magic. You're not told this from the start, and if you invest into Commando on her you'll find yourself lacking a lot of power. Noel has all the great Commando abilities, and has naturally more DMG than Serah. Noel also naturally has more HP than Serah, and more Magic Damage too. Actually, Noel is just is superior in ever way, I suppose because he's a guy -- right? Guys are always better? RIGHT!?
Serah totally lacks key Sentinal abilities too! Like Meta-Guard and Tetra-Defense, which are hands down the best abilities in that catagory. Does she have them? No, does Noel? Yes. Can you unlock them on Serah, ever? No, because no matter how you build her she'll always be the same in the end, weak and heavily dependent on HP or STR items in boss fights.
It's been a long time since I've seen such a purely sexist take on character development, and that really puts a real thorn in my side. Especially during the final boss, when Serah is purposely focused upon at one stage. Can I give her good defense? No, because she just doesn't have it, and she sucks compared to your pet and Noel. Honestly I'd be happier to remove her from the party entirely to maximize my overall performance, because she's more of a handicap than an asset for being so naturally weak.
Ultimately it works very much like a classic Final Fantasy menu -- the only key difference is that instead of picking an ability from the menu you want that character to use that turn, they'll just do that automatically for you. When they've completed the act, you switch to a new Paradigm and repeat. You change based on what you have available and what the battle demands, this is as deep as it gets.
Optional Bosses are very weak really, and offer no real threat when you are max level and spare some potent gear. So all that remains is hunting down Fragments which give you special powers, one of which makes enemies harder (thank god). But you have to pretty much beat the game to get this, and you'll likely have it unlocked after a second play through. By then you'll be max level, and often even tough monsters aren't that hard anymore. I had more trouble with a Behemoth before the final boss than the actual final Boss. Mainly because the Behemoth would constantly pelt all my characters with rapid AoE attacks one after another, rather relentlessly!
There is a lot happening in the battles, so much so that if it was all slowed down and drawn out, a really awesome system would be there just waiting to be utilized. The issue is speed, and right now the battles are so hype-quick you barely have time to register anything. Just tape down your X button and enjoy the show!