When you first enter Deep Dark Wood, you'll get an explanation from Drippy about the use of Waystones, so make sure you use it to restore your HP and MP, as well as save. For now, all you can do is follow Drippy until you get to Old Father Oak. It's time to learn another spell, as well as get your first Familiar, who are little creatures that will help you fight in battles. After you conjure up your first Familiar, Old Father Oak also gives you some more spells, Healing Touch and Fireball.
While you're admiring your new spells and Familiar, some enemies show up and you're thrown into a battle with two of them. After a brief explanation about how to use a Familiar in battle from Drippy, you gain control. Remember that your Familiar and yourself share the same HP and MP, so always keep an eye on it. Once the battle is over, Drippy asks Old Father Oak for something else, but he can't seem to find it, prompting him to ask a favor while he searches for it. The guardian of the forest has become unruly and he wants you to see what the problem is.
WHISPERING WATERFALLS
Once you're done talking to Old Father Oak, head back to the fork in the road and take the other path to Whispering Waterfalls. Save at the nearby save point and start going down the path. Note that this area is riddled with monsters, so it might be helpful to fight some battles to gauge them and use the Waystone to heal yourself.
Head down the only path you can for now, opening the chest along the way (Chocolate). When you get to the fork, the path to the right leads to a chest that's sealed with magic, which you cannot open yet. So, take the path on the left until you cross a log. Follow this path around the bend and before you go through a small cave, there's a hidden path on the right side with a chest (Iced Coffee). Go back down, through the short cave and save, then head into the next area to find the Guardian.
BOSS: THE GUARDIAN OF THE WOODS
This is your first true boss fight and it's not too bad, as long as you pay attention. If you're in close, it will try to hit you with a variety of melee attacks, which should cause around 4-5 damage. However, it's its two other attacks that you have to worry about. When it starts to build up some energy, it will either use Bellow or Windfall; you can tell which one because the name will be above it.
Bellow doesn't really cause damage, but it does temporarily stun you. Windfall is its most damaging move, however, and when it first does this, Drippy will chime in and tell you to defend. This mitigates the damage and also allows some chims to drop. It may also give you an opportunity for free hits against the boss. Once its health gets low enough, a gold glim will pop onto the field. Pick this up to completely heal yourself, as well as gain access to a Miracle Move. Using this should finish off the Guardian.
After the battle, you'll be back in front of Old Father Oak, who will thank you for bringing the Guardian back to its senses. He'll give you the Telling Stone, which is basically like a help section for various things in the game. He'll also give you the Locket, which is needed to cure the brokenheartedness in people, as well as the two spells needed to do so, which are Give Heart and Take Heart. With this, you can exit the forest and head back to Ding Dong Dell.
