Most trophies are quite doable with a fair amount of effort, but Treasure Hunter will require hours and hours and hours of mindless farming.

Completing the story itself will only take up 20% of your time on this game, another 10% to finish the Hunts, and the remaining 70% will be dedicated to battling Adamantortoises and Adamanchelids for Gil, so that you can upgrade all your weapons and all your accessories, in order to obtain this one, single trophy. By using the techniques and guides we linked for you, the Platinum for Final Fantasy XIII doesn't require much skill. However, you'll be looking at anywhere between 100 to 200 hours of game-playing.

As you move through the story, don't bother spending too much time farming for XP, as there'll be plenty of time for that after you've beaten the game and can warp back to Chapter 11. However, always be on the lookout for Treasure Chests to find weapons and accessories you'll need for the Treasure Hunter trophy. Particularly, the Elemental Rings which are rare (note: the Aqua Ring, which many people tend to miss, is found in Chapter 6 - keep an eye out for it!) If you complete the story but missed one of these Elemental Rings, there's still one last chance to acquire it, which is explained in a link below.

Note: We highly recommend you do not sell any of your weapons or accessories, even if it appears you have extras of the same item. The small amount of Gil you'll receive isn't worth the chance of making a mistake, and missing your chance for the Treasure Hunter trophy. Safe items to sell for Gil are: Incentive Chips, Credit Chips, Gold Dust, and Platinum Ingots.

Also, as you work through the story, make sure you're using Libra whenever possible to scan different enemies and learn their full attributes, in order to claim the Loremaster trophy. If you start doing this from early in the game, you should acquire the trophy several chapters before the final boss.

Once you've beaten the story, you'll have the opportunity to warp back to Chapter 11, where the remaining trophies are waiting to be collected. This includes farming to max out the levels of your characters and collecting Gil to purchase upgrades for weapons and accessories, working through the Hunts, and finding treasures using a Chocobo.

Chocobos are unlocked after completing Hunt #14. Try to clear this mission early, not only so you can gain the Gysahl Wreath trophy, but it will make life easier racing around the map quickly by riding these creatures.

The Gold Watch, which is rewarded after completing Hunt #64, slows down the battle time, improving your chances at getting 5-star rankings. This will come in handy when trying to obtain the L'Cie Paragon trophy, so collect it as soon as you're strong enough for the hunt - it will save time in having to replay previous Hunts to obtain a 5-star ranking.

The Growth Egg, awarded after completing Hunt #55, doubles the amount of CP gained in battle. However, you will be doing so much farming to collect Gil, that you'll have the Master's Seal trophy long before you've upgraded all your weapons and accessories. In which case, we don't recommend you go out of your way for this one.

As a strategy, we advise that once you've collected the Gold Watch, start working your way through each and every Hunt, making sure you gain a 5-star ranking for every one before moving onto the next. If you're still having trouble even with the Gold Watch, try using lower-ranked weapons to reduce the difficulty, Fortisol to improve haste, and Deceptisol to quickly stagger the enemy. Do every mission possible, except for Hunt #62! Let's save this one for later.

Next, start farming for Gil. You will need 6 Trapezohedrons (costing 2,000,000 gil) and 5 Dark Matters (costing 850,000 gil) in addition to several other cheaper catalysts, plus purchasing Sturdy Bones (36 will give you the x3 bonus) and Ultracompact Reactors for basic upgrading. If you head to the Northern part of the Archylte Steppe, between the Northern Highplain and Eastern Tors, you will find a narrow space which runs behind a small mountain, where Hunt #63 was found. There, waits 2 Adamantortoises and 3 Adamanchelids conveniently placed near a save station. Simply kill the 5 monsters, save your game, then quit and reload for the creatures to respawn.

Note: Upgrade Fang's Dragoon Lance, Shamanic Spear, Punisher, Pandoran Spear, or a Gae Bolg to a 100 maxed Kain's Lance, then dismantle the weapon, and you will receive 3 Trapezohedrons. This trick also works with Vanille's Tigerclaw, Healer's Staff, Belladonna Wand, or Mistilteinn at a 100 maxed Nirvana weapon. Dismantling a fully maxed Genji Glove will also give you 1 Trapezohedrons. However, there are only 3 Genji Gloves in the game, and we recommend you keep 2 for your battle with the Long Gui. Once you've gotten all the necessary upgrades, head to Bhakti in Oerba to collect your trophy.

Note: While you must upgrade every weapon by at least one tier, you only need 1 ultimate weapon for each of the six team members. Example: Just 1 Nirvana for Vanille, 1 Omega Weapon for Lightening, etc. Now that you have the Treasure Hunter trophy, you're less than an hour away from claiming the Platinum!

All that is left, if you've been following this strategy, is to complete Hunt #62, and battle a Long Gui. We have linked a Youtube video for each battle, displaying the weapons and accessories used in each, which we've found very helpful. It is now safe to sell the weapons and accessories you won't need for the final two battles, in order to make sure all items you need are fully upgraded. We recommend upgrading and keeping: 2 Genji Gloves, 9 Witch's Bracelets, a Wierding Glyph, 2 Sorcer's marks, a Ribbon (or Super Ribbon), a Royal Armlet, a Rainbow Anklet, an Imperial Armlet, a Platinum Bangle (or Wurtzite Bangle), a Clay Ring, and 2 Warrior's Wristbands. For weapons, simply use your ultimate weapons for Fang, Vanille and Lightening. Start with Hunt#62, and follow the guide in the linked Youtube video. Once that's completed, you'll have to save, then exit and reload the game. When you return, all the Adamantortoises will have turned into Long Guis. Again, follow the Youtube video's suggestion (we recommend the 2nd tactic), and you're done!

Congratulations! You've just earned which is probably the most grinding Platinum ever!

For a simple Walkthrough to help locate all the treasure chests, we recommend IGN:
http://guides.ign.com/guides/826843/page_3.html

Here is a map for all Chocobo Treasure Locations:
http://www.gamefaqs.com/ps3/928790-final-fantasy-xiii/faqs/58966

This is a full guide of all the Hunts, including their locations:
http://twobrothersandasister.com/?p=6455

DaveyHasselhoff has put together an excellent Trophy Guide, which includes a checklist for all weapon and accessory upgrades:
http://www.ps3trophies.com/forums/trophy-guides/20108-final-fantasy-xiii-trophy-guide.html

And if you don't feel like writing down all those weapons and accessories, we've even included a link to a downloadable and printable Checklist in TextDocument format, provided by OZ2555:
http://www.mediafire.com/?kzweyztw4go

When you're ready to handle the most difficult Hunt #62, we found this strategy the easiest, provided by MasterLL:
http://www.youtube.com/watch?v=u5ktoPhrPMM

The 2nd strategy of this Youtube video provided by GameXplain should help you easily take down the Long Gui. Also, it helps if you keep an Elixir in your inventory should your battle not go as exactly planned.
http://www.youtube.com/watch?v=fMY6lwyQ3fM&feature=fvst

If you missed any of the Elemental Rings during your playthrough of the story, here is a trick which explains how to turn charms into the desired ring:
http://www.gamespot.com/ps3/rpg/finalfantasy13/show_msgs.php?topic_id=m-1-53881835&pid=928790

And finally, here is a simple Trophy List to browse through:
http://www.ps3-sense.nl/2009/12/trophies-final-fantasy-xiii/