Spirit of the Forest Contract Guide in The Witcher 3 Wild Hunt [PS4 - Xbox One - PC]
The solution for this particular side mission of the game.
Guide Trophy / Achievement "Spirit of the Forest"
To complete the contract you have to play the secondary mission "In the Heart of the Woods" which can be started in Farylund, a small village on the main island of Skellige. Go into the woods and you will come across some villagers, here the quest will automatically start. Investigate the marked areas in the woods and then consult the Spirit of the Forest entry in the Bestiary to continue the mission.
The thing to remember is that you have to accept Sven's proposal and NOT the offer from Harald (the old man), because if you accept Harald's offer you will not be able to complete the contract and unlock the trophy!
The Spirit of the Forest is one of the toughest bosses in the game, for this reason avoid undertaking this mission if you have not passed the level 20 of the character. It has a lot of health and will kill you with a couple of hits, so always activate the Quen sign to mitigate the damage. Thankfully the boss is very slow, so attack him quickly when he appears to keep him from hitting you.
I recommend using a good silver sword that can deal a lot of damage, which you can also improve by using potions like Relict oil. Here is a video showing how to complete this contract, good luck!