You'll need to return to Faron Woods to beseech the Water Dragon's assistance, but something is happening in the woods and you can't travel to most of the destinations there. You'll need to drop into the Sealed Grounds instead - but when you arrive, trouble strikes again.
2 - Defeat the Imprisoned to re-seal it a third time.
The Imprisoned has a tail this time! The fight starts off in more or less the same way as the last battle. Avoid the shockwaves, most easily by stunning it with a blast from Groose's catapult, and take out its feet to disable it for the first jab of the sealing spike. From then on, it's a completely different battle. The Imprisoned will rise up into the sky and start flying! Groose can shoot it down with the catapult, which will drop it back into the pit and give you an opening to drive the spike into its head a second time.
For the third round, things are even more dire. The impact from the Imprisoned hitting the ground from such a tremendous fall has blocked off Groose's supply of Bomb Flowers. He suggests launching you onto the Imprisoned's head so you can attack the spike directly. Aim yourself so your trajectory appears to end right on the monster's forehead, then drive the spike downward from above to finish it off. This time the seal spell is even more complex, but still performed in the same manner - draw the pattern as indicated on screen.
3 - Have Groose launch you into the flooded Faron Woods and meet the Water Dragon inside the Great Tree.
Now that the Imprisoned is out of the way, Groose suggests catapulting you to the Faron Woods. A recent flood has cut off the area from ordinary entry - the old woman was forced to seal the region away to prevent the flood from spreading.
Once you drop in on the area, head for the Great Tree. You'll have to dive down to the lower entry you used to access the tree originally. Thankfully the trip to the top is much easier due to the tree being flooded as well. At the top of the tree's hollowed trunk you'll meet the Water Dragon. She flooded the area due to her wrath towards the monsters and won't retract the waters unless you appease her anger by fulfilling yet another trial. Your goal: collect the Tadtones that make up the last of the dragons' parts of the Song of the Hero.
4 - Track down the Tadtones to complete the Water Dragon's song.
Tadtones are little music-note-shaped tadpoles that when united make beautiful music together. They come in different color varieties to help you figure out which ones should be collected together. You have to gather all of the Tadtones of a specific set or else they'll escape and you'll have to get them all over again. Luckily your spin dash move can nab them pretty easily and they replenish your air meter as well, so this offsets the tedium a little bit.
You'll want to scout out the lilypads off the northeast high dive from the Great Tree before you descend into the waters. You can flip one of the lilypads over to reveal a single yellow Tadtone that otherwise would have been hiding in the coral growing on its backside. This is the most time-consuming Tadtone to obtain, so you definitely want to nab it now so you don't have to climb up the tree again until you absolutely must.
Beginning from nearby the Kikwi elder, dive down into the water and begin your in-depth search. Their locations are as follows:
Charge into the cluster of yellow Tadtones in front of you to get the first eight notes.
Another eight are found on the bottom of the far east section, around a tree. Note that you have to collect them in one clean circle in order to complete the section, so swim precisely and use the spin dash if necessary.
Swim to the west, past the Parella and a series of air bubbles, you'll see the next one, a big purple Tadtone, inside a tree straight in front of you. It tries to run, but it's not too hard to catch, especially if you use the spin dash to chase it.
Underneath the roots of the same tree you'll find four more yellow Tadtones. Start at one end and swim your way through the explosive plants, collecting the notes in one swoop.
Four purple ones patrol around the nearby road leading onto the southern cliff.
Continue going west, past the poisonous bubbles and the Parella, where you'll find another set of purple ones, a grand streak of sixteen notes! It's rough to have to start this stretch over, so use the spin dash if it looks like you're about to miss one of them.
Turn right as soon as you finish collecting them and swim a bit, sticking close to the ground. You'll see two red ones in a grass patch.
North of those two you'll run into a pack of four azure notes.
In the area up ahead there are both a poisonous and healthy stream of air bubbles. Nearby you'll find two purple ones in two grass patches on either side of the path.
Swim north, into the north-west region with the circular path and you'll find four red notes patrolling around.
There's a Froak swimming around a bombable wall. Propel the fish into said wall to uncover a big green one.
Continue east now. A bit above the ground you'll see eight azure ones, and around the Viewing Platform another four pink ones.
Circle along the Great Tree and you'll find a big azure one underneath a root, and a big red one patroling the area.
When prompted by Fi, head to the Kikwi Elder to get the ability to dowse for Tadtones. Now, from the elder's lilypad swim west. At the bottom, through the tunnel you dropped into to find Oolo, you'll find eight green ones.
With the completed song, head back to the Water Dragon and she'll teach you her part of the song as well as drain the woods, this time for good. You won't have to deal with her shenanigans again!
Water Dragon's Song
With this, you know most of the Song of the Hero! Return to Levias to complete the song that will lead you to the Triforce!