You can find the Wood Guardian (Nadaig Frasamen) in Wenshaganaw, “The Singing Waters” to the East of the Missionary Outpost and right next to the Camp near the water shown here on the map. The Guardian is located where the red marker is:
SAVE YOUR GAME before initiating the fight.
The Guardian is large in size, has a ton of health and armor and is VERY resistant to magic. Magic attacks hardly do any damage so if you’re a mage I highly suggest using firearms if you can to lower its armor to help your companions DPS. Also note that the Guardian may be surrounded by bandits when you come across it. The bandits are rather weak so take them out first before focusing your attention to the Guardian.
The Guardian is extremely tough and resistant to magic, so plan accordingly. It’s attacks hit quite hard and can one hit you if you don’t have enough health or armor before fighting it. The Guardian will occasionally leap a large distance and damage / unbalance you or surrounding companions if it lands near you. If you see it jump, dodge before it lands. The Guardian will also summon a sort of telekinetic fire circle around itself and sometimes around you if you’re not careful. If you see yellow and red rings forming, dodge and move immediately before it explodes. If you’re attacking with melee also be aware of the sweeping attack the Guardian will do with it’s arms. This can throw you backwards and leave you exposed for one of it’s other attacks before you can recover. This is a tough fight and I found myself using Stasis and then spamming my firearm to bring it down. Pay attention to the attacks, dodge, and keep chipping away at the HP and you’ll be victorious.
If you’re having difficulty fighting the Guardian you can change the difficulty to Easy in the game settings.
Once you kill the Guardian this achievement will pop. You can loot it for an item called “Horn of the Forest King” that can be sold to merchants for 150 gold. There’s also a dead body nearby that has a key to a chest that you can find close to the camp near the Guardian location. The chest has a full set of unique legendary armor inside.