In the Heart of the Woods is a Side Quest that can be started in Fayrlund, a small village on the main island of Skellige.

Go into the woods and you will run into some villagers. This will automatically begin the quest. Investigate the marked areas in the woods and then look at the Woodland Spirit entry in your Bestiary to continue the quest.

Accept Sven's offer who wants you to kill the Woodland Spirit (the younger man) and not the offer from Harald (the elderly man). If you go with Harald's offer you won't get to fight the Woodland Spirit and will miss this trophy!

The Woodland Spirit is one of the hardest bosses in the game. It has a lot of health and will kill you in 1-2 hits. Always activate the Quen Sign to protect yourself from damage. The boss is quite slow though. Attack it quickly when it appears, so it doesn't get a chance to attack you first (and if it does your Quen Sign will protect you).

Before going into this fight you should get a silver sword with good damage stats and use Relict Oil on it for some bonus damage.