XIII-2 newbie here; I'm currently in the Bresha Ruins, and I just got the ability to tame monsters. I have to say that I love the game so far, and I can definitely see how this one is much more open and has loads more to do compared to the original game.... but the huge jump from linearity (which I did not mind at all in the first game, by the way) to almost no linearity is somewhat overwhelming me.
I'm slowly starting to understand the Crystarium better thanks to the Crystarium Progression FAQ, but I'm not sure how to go about leveling up monsters and/or picking the best ones, and I'm guessing there's a trophy for getting all of them, right? And I know I shouldn't be worried about missing doing a quest or getting an object now that I know you can redo the area as many times as you want, but it's hard not to be paranoid. Like I just found out that apparently I've already messed up getting one trophy, since I failed the Cinematic Action sequence during the Atlas battle.
Any tips on what I should be focusing on primarily would greatly help soothe my nerves, lol.
There's only one missable thing in this game (aside from character stats), and it's an irrelevant adornment. You can fight Atlas again.
It's pretty hard to pick good monsters without experimenting. Note that early peakers and well grown monsters level easier than late bloomers, and thus are often better. As for leveling, just use whatever you've got. These early materials are easy to come by later in the game, so you don't need to feel like you are wasting them. You'll mostly collect Potent drops, so that's what most of your monsters will be using for now.
Note that Com monsters are really good. So are Syn monsters and Rav monsters. Keeping a decent Sen monster around will help for harder encounters. Med monsters are helpful until you unlock Med for Noel and Serah, and then they are useless. Sab monsters are always useless.
Etro's Gate---The gameplay nexus for Fabula Nova Crystallis Final Fantasy http://etrosgate.com/
Cait Sith and Zwerg Scandroid can actually last you quite a while.
If you plan to stick with Cait Sith as your Medic and he hasn't learned Raise but the time you get to Yaschas Massif -010 AF-, then find a monster there called Spirathes and level it up to level 7 and it will learn Raise. You can then infuse it into Caith Sith so he can revive people. In fact if you level Spirathes all the way to 40, it has a lot of useful moves you can give to Medics.
Flanitor is generally the best Medic.
The best Sentinel people say is often Goblin Chieftain from Archylte Steppe.
If you watch "Doctor Who" and play Pokemon Platinum, this will blow your mind- http://img43.imageshack.us/img43/6649/1262538638159.jpg