Unlike what a previous poster said, the only required battle before the rose is the Silver Sentinel (you have to defeat him to get across the bridge).
Once you have done so, open up the map. In the bottom-right corner there is a tower. That's where you need to go next (you'll have to open the "Gate of Despair" to get there).
When you reach the tower, and you are facing it, go to the RIGHT SIDE of the tower and look there. At the very back corner there should be a rock, check behind it to find the rose.
