With Dungeon Man's help, you are able to reach the Deep Darkness.
Teleport if Needed
The Deep Darkness is in Ness (and Poo)'s Teleport list now, so you can teleport away if needed. If you don't have enough room, wait until you reach the northeast area nearby.
Get the Hawk Eye
If Poo had the Hawk Eye before he left with the Star Master, you got a call from Escargo Express earlier. If you didn't get the Hawk Eye from Escargo Express earlier, you can use the nearby bird phone to call them now. It's free.
If you bought the Piggy Nose in Scaraba and left it with Escargo Express, get it now, too.
Talk to the nearby monkey to heal up for free.
If Jeff's IQ is 45, he can repair the Broken bazooka that you got in the Fourside Sewer under the museum. It becomes the Heavy bazooka, which does the same damage as a Super Bomb, but you can reuse it as many times as you want. It is a very helpful item to have, so if Jeff's IQ is almost 45, you might want to level up until his IQ is 45, then sleep in the monkey hotel (or some other hotel) until Jeff repairs the Broken bazooka.
If you want to collect all of the treasures in the Deep Darkness, make sure to have at least twelve free spaces in your inventory.
Just east of the bird phone and the monkey hotel, there is shallow water, then some deep water east of that. You will have to walk in the deep water, but you will gradually lose HP while you walk through it. Keep an eye on your health as you go.
When you first enter the deep water, go southwest in the deep water and use the Piggy Nose to find a Magic Truffle. If you are close enough to the Magic Truffle, the Piggy Nose will tell you which direction to go to find it. When the message from the Piggy Nose says "It must be right here!" press L to check the area and find the Magic Truffle.
Then go east to reach dry land. You can buy Multi-Bottle rockets and other useful goods from the arms dealer, but don't get the yo-yo that he sells. It misses 18.75% of the time.
Use the Piggy Nose north of the tree near the arms dealer to find another Magic Truffle.
Go north of there, then talk to the monkey next to the tree and say Yes to teach it Teleport. The Monkey will give you an item called Monkey's Love, which you can use during battle to solidify the enemy temporarily. Give the Monkey's Love to whoever has the highest Luck stat.
You can talk to the businessman to the left to heal up for free (the "heal" option restores your HP and PP). Just agree to the fee, but choose "Let him pay". Never choose Cash after that or you will have to pay him back. If you let him pay, it adds some dialogue to the game's ending.
This area has enough room for you to Teleport elsewhere, if you want to. As mentioned before, the Deep Darkness is in your Teleport list now, so you can teleport back here when you are done.
When you are ready, go south from the arms dealer. Soon you will reach an area that is very dark. Use the Hawk Eye while standing in this area to light it up. The Hawk Eye disappears from your inventory after that. Open the nearby gift box to get a Banana.
Go south from there. You can talk to the snorkel to get cash from the ATM. However, the snorkel man also charges a fee that is the same amount as your withdrawal. So if you get money from him, you will have more cash on hand, but you will lose twice as much money from your bank account. For example, if he withdraws $100, you will have $100 more on hand, but you will lose $200 from your bank account. Since the snorkel man is near the beginning of the Deep Darkness, you might prefer to just teleport somewhere with a free ATM and then teleport back to the Deep Darkness after.
Continue going south from the snorkel. When you see some dry land on the left, go there. Then go northwest from there. After you go through some deep water, go west to some shallow water and open the gift box there to get some Beef Jerky.
Go northwest from that gift box and walk into the left side of the deep water against the trees, and use the Piggy Nose to look for a Magic Truffle there. Then go southwest from there and open the gift box to get an IQ Capsule. If you walk to the right of the nearby bird, the Photo Man will take your picture.
Go east and then south in the shallow water. Then go south in the deep water, then west when you can. Go north from there and open the gift box to get a Cup of Lifenoodles.
Then go south, and go west through the gap in the trees. Continue going west through the long expanse of deep water. On the left side, use the Piggy Nose just north of the western shallow water to find another Magic Truffle.
Go southwest from there until you find a gift box that contains a Souvenir Coin. Go north from there, then go east through the deep water north of the trees. Go north when you can, and open the gift box there to get Rock Candy.
Go south from there, then go southeast in the deep water and follow the path. Soon you reach a boss. Talk to it to start the battle.
Master Barf has 1319 HP. This boss is NOT weak to Fly Honey, so don't use it. If Jeff has a Multi-bottle rocket, use it to end the battle quickly. Otherwise, use PSI Rockin, Freeze, and Jeff's Heavy Bazooka if he repaired it.
After the battle, do NOT equip the Casey Bat. It misses 75% of the time! Don't sell it or throw it away, though. It can be useful.
After this, go north. There will eventually be some shallow water to the right. Walk north in that water until you see some shallow water to the west. Go to that shallow water. Then go north from the east edge of this shallow water and use the Piggy Nose to find the last Magic Truffle.
Now just go west. When you reach dry land, go west and then north to find a cave entrance. Go in.