For this trophy, you need to defeat the hidden boss Raspatil on Normal Mode, so don't switch to Easy Mode to do this fight.
Raspatil can be found in Oerba 400AF at the top of the stairs that lead down to the beach. Make sure you have Battlemania turned on so Raspatil spawns quicker (Battlemania is a fragment skill that can be obtained after 100% exploring all maps). Before fighting Raspatil, you'll want to make sure that Serah and Noel are either completely or almost completely leveled up in the Crystarium (I recommend having Ravager, Commando, and Medic complete for both of them, and have Serah be mostly through Saboteur). Also, take some Wound Potions and Phoenix Downs into the battle with you just in case.
The monsters that you'll be using should be decently leveled Synergists, Commandos, and Ravagers. Personally, I used Yakshini (SYN), Narasimha (COM), and Koboldroid Yin (RAV), but these are not the only ones that can be used to good effect. Gahongas is another good Synergist because of his feral link (I've also heard that Purple Chocobos are the best choice for this fight), any Commando that does good damage and has decent vitality will work, and as long as your Ravager's vitality is decent then it works too.
Set these as your paradigms:
Cerberus-X (COM/COM/COM tuned to Cross)
Tri-Disaster-X (RAV/RAV/RAV tuned to Cross)
Rapid Growth (SYN/SYN/SYN)
Start off with Espionage and try casting Deprotect, Deshell, and Imperil. If these don't stick on Raspatil, just put either Deprotect or Deshell and then move on. Switch to Tri-Disaster-X next and build up the stagger meter. As soon as Raspatil is staggered, have Serah use Ultima Arrow to boost the stagger meter higher. You should try to get as close to 999% as you can, but if it's taking too long, then it's okay to move on once you hit about 700% or so.
When you're done boosting his stagger meter, switch to Cerberus-X and destroy his health meter. When the stagger meter is almost empty again, have Noel use Meteor Javelin for some extra damage, then go back to Espionage. Rinse and repeat.
Of course, it's not that simple, but that's your general outline. In certain cases, you'll have to change your strategy a bit:
- If Raspatil calls for allies, immediately switch to Cerberus-X and take them out as quickly as you can. You can't really afford to leave the other Cie'th hanging around.
- If your health starts getting low, then switch to Convalescence and heal as fast as you can.
- If you start getting too wounded and your health meter isn't big enough to be sustainable, use a wound potion.
- If someone dies, don't give up. Switch to Convalescence and use a phoenix down, then heal and keep going. I had my monster die twice during my try at this trophy and I won comfortably at the end.
- If Raspatil uses Aleph Zero, switch to Conservation immediately and try to guard it. If you don't guard it, don't worry, you'll take less damage anyways due to having two Sentinels active. You'll probably need to heal your monster afterwards as well, so go to Convalescence and heal him a bit.
Here are some general "Don't"s for the fight:
- Don't try to poison Raspatil. He's immune to it so it won't work.
- Don't waste too much time between staggers trying to cast debuffs. They help, but Raspatil will most likely call out more Cie'th if you take too long getting all three of the important debuffs on him.
- Don't ever let the additional Cie'th stick around more than they need to. They will quickly kill one of your party members if you let them linger too long, and they can help Raspatil in other dangerous ways.
- Don't get impatient. This is a battle of patience. This won't be a quick battle for most people, especially if your party members aren't fully leveled up, so just play it smart and you'll live through it.
- With that said, try not to let the battle drag on way too long. The Cie'th that Raspatil spawns get stronger as the battle goes on, so make things easy on yourself and don't keep fighting him for longer than you need to.
When Raspatil is slain, you will get tons of CP and a nice gold trophy for your troubles. Good luck!