Many players run into an immediate problem inside Jabu-Jabu. They defeat the enemies, but the door is shut. To open the door, you must shoot a slingshot pellet at the thing hanging down from the ceiling. You need to remember this solution elsewhere in Jabu-Jabu's stomach. Another problem is Princess Ruto. You have to speak to her a few times before she consents to you carrying her throughout the level. However, if you drown her or forget her, she will always reappear in the spot where you first found her.
The "big, slimy things" also pose a problem for most players. To defeat these, you must defeat their tails which are in other rooms in Jabu-Jabu. To defeat the tails, you must lure them down from the ceiling by walking close to them (while you are doing this, Z-target with your boomerang) and when they come down to hit you, back away and throw the boomerang at the same time. If you time it right, they will change color and will be damaged. Repeat this method until they die. When a tail dies, a big, slimy thing will disappear allowing access to the rooms behind them. There are also, some red, spongy looking things that wobble to attack. Throw your boomerang at them to stun them temporarily. During this time, you can use them as stepping-stones. Another problem is the switch which will not go down under your weight alone. To trip this switch, pick up the princess and carry her with you onto the switch, and then place her on the switch to hold it down while you go into the room.
There is another switch in the level that needs a wooden crate to be placed on it as a weight. There is an octopus mini-boss in Jabu-Jabu's stomach. To stop his wild charge at you, hit it with your boomerang to stun it. Then wait until it starts spinning around in place and hit it with your boomerang again. If you time it right, he will be facing the opposite direction. Strike at the round thing on its tail with your sword when this happens. Repeat this procedure until the octopus dies - then jump on the circular platform and it will raise you up one floor. There is one last puzzle that stops players in the room right before you meet the boss. There is a switch hanging from the ceiling, but you cannot shoot it with your slingshot because there is a spider web blocking it. To solve this puzzle, back up a few feet and throw your boomerang. It may take a few tries, but eventually the boomerang should trip the switch and allow you access to the boss. The fight with this boss is rather long and difficult.
Here is always a good method to go by: if you find a special item that can be used as a weapon in a level (like a boomerang for example), it is VERY likely that you will need that item to defeat the boss.
This policy holds true in this case. At the beginning of the battle, dodge the electric bolts and throw your boomerang at the enemy. If one of the little parasites drop to the floor, attack it with your sword. When all of the parasites have been defeated, the boss will try a new method of attack. It will swing the parasites around in circles. Try to time it so that your boomerang slips between the revolving parasites and strikes at the main body. When this happens, everything will momentarily stop. This is the time when you should strike at the parasites. Defeat these and then go after the main body with your boomerang and sword. It may switch back to an earlier attack method, but eventually you will wear it down and destroy it. When this happens, pick up the heart container and exit via the glowing light. The princess will give you the stone - your next stop should be to get the Master Sword in the Temple of Time in Hyrule Castle…
Proceed to The Master Sword...