The Guardian of the Woods is located in the Deep Dark Woods near the south. This guy is a pretty simple boss. He's weak against Fire attacks and is resistant against Storm attacks.
He has 3 attacks: a physical attack, Windfall and Bellow.
My advice is to use Oliver almost exclusively and use his Fireball spell as much as possible. Make sure to stay far away from him, and when the Guardian uses Windfall or Bellow, make sure to Defend.
Monitor Oliver's health and mana, and if he's low on either use a Healing spell or Iced Coffee, respectively.
It's as simple as that! See the YouTube video by JRPGGamer for more details.