I'll start by saying I'm a completionist. If anything is missable or can be messed up in anyway then I'd like to know. Beyond that, what guide here would be best to follow treasure collection wise? I prefer guides with item checklists at the beginning of each section if there are any. Beyond that, I'm willing to listen to any advice the board has.
With each passing day, the world finds new and exciting ways to kill a man.-Balthier FFXII
There are no actual missable things in this game, because you can enter an unlocked area at any time and even, with the proper Gate Seal, wind time backwards and replay the Storyline parts of that area.
Maybe look at the Monster Guide to see where and which ones the One-Time-Only monsters are.
Cloud and Sephiroth - battling each other since '97 for your amusement Give it a rest already!
Depending on how much of a completionist you are, you might want to avoid the one-time monsters that you get outside of battle. You can pick them up from Valfodr and then get them from Mog/ a treasure sphere to have 2 of them (or four for the chocobos).