Let say: If a player wants to get the Bow of Crags, he will not be able to get the WHOLE weapon in the loot, instead... at the end of each trail each player will get a random item that he can use to create the Bow of Crags. Let say the recipe for that bow could be:

Bow of Crags
Titan Ore (x3)
Cinder of Ifrit (x5)
Ancient Bow (x1)
Heart of the Crags (x2)

So in this case, after every fight each player will get a single item, (randomly generated), if he got a multiple copy of the one he doesnt need then he plays it again, if he got everything then he has the bow. The items can be even from different trials, so that people will have reasons to play it again and again. They could even have a trading system, wherein you can trade your extra items, to items of the same rarity (no gil exchange of course only item to item trade off.)Something like that...