Every character in the game has an ultimate Spritnite stone that can be found. Right before you have to fight the last boss, there is a point in which you can roam the world. A portal will open up and you can easily get back to the last boss.
This particular trophy is to get the Gearadia Spritnite for Aeterna.
This is a pretty easy stone to get. No boss fight involved, you just have to follow directions.
The following link was very helpful to me for collecting all of the ultimate Spritnite stones:
http://teambrg.com/i-am-setsuna/setsuna-character-sidequests...