A slimy eel will release Shy Guys and you must swallow them for egg to throw back to force the slimy eel at bay. A Red Coin is by a set of Mecha-Piranhas. Descend with the ledges and turn left to find 2 Red Coins by a Lakitu in the water. Slide downhill and defeat some Piranha Plants to enter the next area. Bounce an egg off the wall and let it bounce on the water so it collects 4 Red Coins. Under another hint box, there are 3 Red Coins on the left and 2 Red Coins on the right with the first flower concealed in a ? Puff. Where Piranha Plants appear, the 2nd flower is on the left and the 3rd Red Coins are on the right. In the next sewer room, the 3rd flower is by some Ravens. The checkpoint is between 2 Piranha Plants. Go into the water and left until you spot a massive slime block monster. Throw eggs at it to make it shrink. Continue left and you’ll find 3 Red Coins above Mecha-Piranhas. The path top right from the checkpoint has the 4th flower by a Piranha Plant. Take the lower right path and shrink a big fluid monster blocking the way. After 2 Piranha Plants, you’ll find the last flower with 2 Red Coins. Enter the red door and you’ll battle Naval Piranha. This boss will use its tentacles to lunge and use its head to charge to the other side. Throw eggs at the water so it skips and hits the belly of the boss. Hit the belly with eggs 3 times and you’ll receive the key to the next world.
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https://www.youtube.com/watch?v=sBC6_0rbZ78