Ugh, the legendary XIII Platinum. Broke my back on that one. Couldn't play the game for months after that. I did have some fun with it though, including the 25 hour marathon where Splint kept me company all night. I was on Skype with Lex a lot when I did that trophy, grinding toises until I was green in the face. I suggest having a plan for how you spend your gil. I had written down how many Platinum Ingots I needed, but I always ended up needing 5 more, and 5 more, and 5 more... that's because I brainfarted on a few things, plus I got this insane wish for leveling 2 of someone's weapons, probably Vanille. 'Ohey I'm already so much into this I can do one more Tier 3!' No. Don't do that. X) Also there are smart ways of selling stuff you no longer need, like dismantling some Tier 3 weapon into other weapons, or how that was. The downside is you'll no longer have a Tier 3 weapon for one of your characters. But that won't be a problem as you'll not want to play the game for some time anyway after popping the trophy. X)
Some fights I had to redo several times, others I got on my first try. Beating the final target gives you a Gold Watch which lets you 5 star more easily, so if you're having problems I suggest go do that. You need the Gold Watch for Treasure Hunter anyway, and you can't 5 star the last mission without it. (Or, I guess there is a way, but most people can't.) One of the hardest mission is those two floating shielded fuckers on top of a rock on the Steppe. I'm just saying, regarding that fight, and a lot of other fights too: Equip magic resistant accessories! And look into which element spells/ moves are based on. (Hint: Earth.) I could never have beaten Long Gui on my first try if I didn't dodge Ultima the way I did.
I did Treasure Hunter as my second to last trophy. I saved the final hunt so I could have the hunts (hunts? this is XIII Fangu; missions, yes) be the last thing I did. So when I finally 5 starred him, both trophies popped. And then I could climb out of my sofa and go back to having a life.