I did this on level 61, straight after saving after fighting the boss.
Preparation:
* You MUST have two Save books and three Secret Room books. You can find Save books anywhere, but I didn't find Secret Room books until around level 50 or so. I even had to duplicate the Secret Room books with the Notebook to get up to three, so keep in mind that you may want to hold on to a Notebook or two if you find them before the Secret Room books.
* You'll probably want some Hope books as well to resurrect you if you get Doomed.
* Dark Knight dress sphere, of course!
* Doomproof accessory. Make this beforehand by wearing Dark Knight/Trainer/Samurai. If you don't have the accessory, wear the dress spheres. If you don't have those, bring a ton of Hope books, because Doom sucks when it's used on you.
Now, for the solution!
As soon as you hit level 61, use the Secret Room book. If you end up in a room with a bunch of stones in a circle with a stone in the middle, hurray! That's the right room. If you end up in a room with a bunch of enemies, reset and try again.
Now, break the stone in the center. This unlocks the elevator and opens a room with an item. Take the item (usually something decent), and SAVE on the elevator.
Now, use your second Secret Room book. Again, if you end up in a room with enemies, reset. If you end up in a room with more stones again, you're in the right place. The stones are in a sort of X pattern, with a stone in the middle. Break it to unlock the elevator and open another room with an item. Grab the item, and SAVE on the elevator.
Last one. Now use the Secret Room book again. If you end up with enemies, reset. You know the drill. If you end up with stones, you did it! The stones should be in a triangle. Break the stone in the center to unlock two rooms. Get the item, then go in the next room to trigger the fight with the Tonberries.
Hopefully you read my other solution and are set up as a Dark Knight - if so, this battle's a breeze. You'll fight four Mega Tonberries and one regular. Don't even bother trying to go head to head with the Megas - they have too many HP, and too much defense. Just get each one in range, Doom it, then run away. You'll have to wait 45 turns for each one, and you have to be kind of close for the counter to actually work, but it's still faster than engaging them in battle.
An elevator will open up mid-battle. Don't use it! Make sure you kill everything and get the item that appears at the end. Congratulations... this annoying trophy is yours!