Serpent Road lies in the deepest portion of Radish Woods, near the northwest-most corner of the map. When Parin enters, you'll see twin rows of burning torches and ahead, one of the challenge statues so fond of making you meet criteria before you can enter. In this case, you must light the eight torches that line the road. To do this, stand to the side of the rows of torches with the Fire Parts equipped. Face toward them and dash guard forward toward it, then rotate the analogue stick as you arrive nearby and press 'X' to attack with your special move. It should send waves of fire forward to light some of the torches. Make sure you don't get close enough that you destroy them. You'll have to repeat the process three or four times at least to gradually light all of the torches. If you mess up and accidentally destroy any, you'll have to leave. Once you succeed, talk to the statue and he'll open the door so that you can proceed.
From the entrance to the next area, you'll see three Phantoms conferring to your left. Approach them and use Drill Tornado and charge attacks to make quick work of them. You can also destroy a tree there. Then proceed down the narrow gap in the rock wall to the right of where you first saw the Phantoms. You'll be able to jump up the short ledge there, where you'll see a dirt mound near a tree. To the right are several pots you can break open. Do so, then destroy the tree with your drill and make use of that mound of dirt if you need it to power up your drill. If you're not already wearing them, switch to your goggles (or a powered-up Nightcap, if you have it) and jump out over the water to the right. There are fish swimming there, so take care of them before you swim to the right bank. One of them is a large eel and will take quite a bit of abuse before he'll surrender.
Once the fish are gone, head to the bank and break four vases, then jump back in the water and swim down the stream as it continues deeper into the forest. You'll see another bank to your right, with some ledges that lead up to a lever. Climb up to it and pull it to cause some enemies to appear over the water to your left. Drop down to the lower bit of the bank, then jump and use targeted attacks first on the bottom enemy, then on its fellows as you use them as stepping stones that lead past a watery area where spike-tipped pendulum traps are swinging back and forth. There you'll find a ledge with three pots and some stone pillars. Break open the pots and then pull the lever, which will open the gate on the first bank you encountered near the stream (the one you climbed to after killing the fish). Now use the floating red enemies as stepstools again (they'll have reappeared) to make your way back toward the gate you opened and you'll have to defeat some walking plant enemies that materialized when you caused the gate to open. Don't accidentally pass through the gate before they're properly dead or you won't get the 'S' rating at the end of the dungeon. Once they're defeated, pass through that gate and into the next area.
From where you enter here, you can look ahead to see several plants that you must defeat, as well as a pot and a vase. Destroy the plants and pots (and drill the tree if you need some coins), then proceed down the canyon. The path splits here. Start by taking the path leading left. There's a purple vase to destroy along the way and then you'll come to a fallen log. Walk left around it and you'll find a tree you can drill as well as a pig-shaped pot. Bust it open for a lot of coins and you should notice that beyond it, there's a weak segment of wall you can drill. Do so to reveal a small cache of coins. Now return to the path and continue along it as it heads left. When you arrive near the torch, two armored Phantom enemies will drop to attack. Hit them with short-charged drill attacks to send armor flying, then finish them off with a few more such strikes. Now continue past where they appeared and break a vase before following the path as it curves sharply left and then back right. You'll come upon a new clearing. To the left and right, you should see some plant enemies. Defeat them quickly and also dig in the mound of dirt if you need it.
Continue along the trail and bust the two purple vases as you do. The path curves slowly to the right, and comes to another clearing. Here, there are Phantoms snoozing to your right. Attack them with charged drill attacks and make sure that you don't miss the ones hiding in the shrubbery or your ranking for the level will suffer. Don't head through the dark opening just yet. Instead, backtrack along the trail until you come back to that first 'T,' which you'll encounter now shortly after passing the fallen log. Continue to follow the path as it heads toward what is now your left (but would've been the right fork when you first came to the 'T'). There's a torch burning near a pot. Bust open the pot and torch for coins, then drill the weak wall behind where they rested to reveal a new opening into a clearing.
As you head into the clearing, you'll find yourself on a raised ledge. Jump out onto the forest floor below and use your drill on the various trees in the area. They leave coins behind. Destroying many of them also causes Phantom enemies to drop, one apiece for the majority of the trees. These can be defeated with a few charged drill attacks and you'll get some Junk as a reward, too. When you've destroyed every tree, make sure you have the goggles or a powered-up Nightcap equipped and jump down into the well.
At the base of the well, there are some submerged pots you can break. Jump into the air over the water, then use a downward drill attack to break them. Now head to the edge and there are ledges you can use to climb along the well, to a weak wall that you can drill to pass through. You'll be able to follow a stone path now as it winds its way up and round the well, to a ladder that leads back up to the surface. At the top you can break another pot and there's a lever you can pull that drops a nearby gate so you can proceed through the forest.
Head along the path and take a left, then follow it through a canyon and you'll come to a sign advising that you're on a mistaken path. Push past it and break the pot you find there, then head back so that you're facing the back of the sign referencing the mistaken path. Continue along the path beyond it and watch the wall to your left. There's a recess where the wall is cracked. Drill it to reveal a small clearing with a treasure chest surrounded by a bunch of lavender-hued pumpkins. Defeat them with a Drill Tornado attack or two and open the treasure chest to obtain POWER OIL. Also in this area is a container you can bust open for some Junk and a pot you can break. Now head back out of the opening and continue along the path to your left. Follow it back through the clearing with the well, and keep heading along that path until you follow it back past the 'T' and back past the fallen log, all the way to the black opening that leads to the next segment of forest.
You've reached the treasure room. Jump up on the altar to grab the BOOKSHELF. You should also win a gold medal for gaining an 'S' ranking, naturally. Take it back to Poco in Monster Village. He'll be grateful for its return and will cause more Dark Mist to vanish from the map, north of the stage you just completed.