Ocean's Roar Cave is the fourth dungeon in Dark Cloud 2 and takes place during Chapter 4: Goodbye Shingala. It is located in the Veniccio coast, and contains floors set in underground sea caves and above-ground coastlines. The intermediate floors contain only underground portions, and so spheda may be played here. The gate key used is the Luna Stone, and getting through blocked paths requires the Luna Piece. The Ocean's Roar Cave introduces the Aquarium and the Finny Frenzy to the player.
The floors on Ocean's Roar Cave are huge, but there is ample time given to defeat all monsters and collect ABS and Gilda. The time trial medals are especially easy to get if you have weapons that can one-hit kill and have Pau in your support character slot. You should even have time to throw all the rocks in the level to look for items, assuming that you don't dawdle around unnecessarily.
As always, bringing along Improved Bombs kills many of the enemies in the item-only stages. Enemies that are particularly hard to hit such as the Sand Molers can be weakened with Poison Apples, and finished off using Bomb Nuts. Undead enemies can be weakened with Holy Water, and may be finished off using other items. Pirate Tanks can be highly annoying due to their tendency to use cannonballs, which can screw up your chances of earning an item-only medal. Using a Stone Berry or a Heart-Throb Cherry, and finishing them off using Improved Bombs is the best way to bring them down.
Spheda in Ocean's Roar Cave is easy, as there are rarely any off course areas, and only completely-underground levels have spheda medals. Do note however that the cave walls are uneven, so do not try to bounce the ball off the wall and expect consistency. Occasionally you may get the ball stuck right next to the wall, in which case you may move the ball using the square button to a nearby location, but this will cost you one shot. Also note that the ball will lose most of its momentum when it touches the floor, due to the floor being covered in seawater, so using topspin may help counteract this. Otherwise, this dungeon is one big playground for spheda, with the Swan or Flamingo being generally the best clubs to use.
Only the levels that contain above-ground portions have fishing medals, as fishing can only be done by the coastline. But like any of the fishing goals, it requires patience and/or luck, and can take anywhere from a short to a long time. Consider investing fishing points in your Fishing Rod or Lure Rod to make things easier.
You need to first advance in Veniccio's georama before you're able to take on Shingala. Talking to Osmond in the future will eventually prompt you to get an Aquarium and an Electric Worm. Feeding the electric worm to any fish will transform it into an Electric Fish, which will be kept in the tank until you're ready to fight Shingala.
For the boss battle with Dr. Jaming, see Dr. Jaming.
Visiting the Shigura Village for the first time will not prompt anything. Once you advance in the georama and receive the Shell Talkie from Dr. Nobb in the future, you will be able to communicate with the shigura and receive the Secret Dragon Remedy for Shingala's injuries. After giving the medicine to Shingala, Gaspard's flying battleship will be heading to the village. Returning to the village, you will find that Dr. Jaming has placed mind control devices on the shigura, and he must be fought.