You need the airship to access this one. Once you have it, there is an island in the north west that contains "Morthshaw Woods". It is a small island with trees. Easiest way to locate it is look for lots of ice floating in the water on the west side of the main island. Follow this west and slightly north and you should find the island.
Proceed into the woods and you will find what is needed to access this and get the trophy