The main purpose of completing Twilight Princess's Cave of Ordeals is to release fairies into all five fairy springs that are spread throughout Hyrule. This helps Link on his quests, and is especially helpful during the battle with Ganon. The trick to this is that high-end items are needed before this side quest can be complete, making its benefits only applicable for end-game adventures.
It's still worth it though, especially since the main reward is Fairy Tears. These go inside a bottle and will replenish Link's health, also boosting his attack power as long as his hearts stay full. Between the fairies and the Fairy Tears, it's a good idea to complete the Cave of Ordeals before facing Ganon.
7 Floors 1-10: Ordon Spring
In this section, Link will fight everything from Rats and Keese, to Skulltula's and Lizaflos. This section is obviously the easiest, but it's still good to be prepared. As a reward for completing the first ten floors, the Great Fairy will release fairies to the Ordon Spring.
There will be enemies like Torch Slugs that need to be knocked down from the ceiling. It helps to use the hookshot to knock them down to the ground, then proceed to slice them till they're dead.
6 Floors 11-20: Faron Spring
From here on out, Link will need the Spinner to proceed. This item is found in the Spirit Temple. Floor 13 has those disgusting worm bubble enemies. To defeat them, use the claw shot to pull them from their protective bubbles, and get to slicing. On floor 14, Link can turn into Wolf Link to find 100 rupees in the ground. These are found by using the "dig" move. On floor 17, there's a poe, which is essential for the poe side quest.
If Link is getting low on hearts, but players don't want to waste fairies or potions, dig around as Wolf Link to find at least one heart on floor 18. The Great Fairy on floor 20 will thank Link by releasing fairies to the Faron Spring.
5 Floors 21 Through 30: Eldin Spring
In order to continue, Link needs the Ball and Chain weapon. This one is obtained in the Snowpeak Temple. As one would assume, these floors are filled with lots of ice enemies. Use the Ball and Chain, bombs, and Fire Arrows to navigate this icy portion.
Floor 24 contains several Re-deads. Yikes! There's another heart underground, which is located on floor 27. Don't forget to use Wolf Link's senses to find hearts and rupees. Once Link reaches floor 30, the Great Fairy rewards him by releases fairies into Eldin Spring, which is the spring in Kakariko Village.
4 Floors 31-40: Lanayru Spring
Now players are getting a mix of all the previous enemies combined into one battle. This starts to get complicated. Link needs the Dominion Rod to continue from here. On floor 38, much of the work can be down before even jumping down onto the main floor. Simply use the Boomerang to destroy those pesky Ice Keese, and use Bomb arrows to kill the Chilfos. This will save a lot of on-ground combat, saving some hearts.
Definitely prepare for floor 39, where two Darknuts appear. These are not very easy to fight. Make sure to not trigger them both at the same time. Try to fight one while the other stays still. If they both come after Link, it won't be a fun time. Don't forget to use Midna to turn into Wolf Link to dig up 200 rupees in this room before leaving.
The Great Fairy will be grateful, just like all the others, and will release fairies into the final Lanayru Spring which is located at Lake Hylia. Remember the place with that creepy cutscene where several weird-eyed Links appeared? Yep, that's the one!
3 Floors 41-50
Now for the hardest section. The Double Clawshot is required for this portion of the Cave of Ordeals. Level 41 starts out strong, so don't expect an easy ride. There are several Armos awaiting. Just like Darknuts, it's best to not alert more than one at a time if at all possible.
Further, floor 43 has Archers and Lizafols. To wipe out the Lizafols quicker, it's best to use arrows to combat the Archers before hopping down below.
2 The Hardest Floors
Floor 45 is an absolute mess. There are multiple enemies, and it's easiest to take them all out in a certain order. Be careful, because three Archers will shoot at Link as soon as he walks through the door. Have the shield ready. First, start with the Archers from above, just like on level 43. It's best to stand back in the doorway a little so they don't destroy you as easily. Then, use the Ball and Chain to take out some Re-deads. Now all that's left is gooey Chus.
Floor 46 is even worse. Make sure to turn into Wolf Link to eliminate the invisible rats that hold him down. Floor 49 is the most difficult by far, which is expected since it's the final floor. Three Darknuts are awaiting on this level. It's best to take on one at a time as always, because three at once will be too much. Try jumping down to the left lower end to attract one single Darknut and battle it without getting too close to the others.
1 Final Reward: The Fairy Tears
The reward is plentiful. The aforementioned Fairy Tears now belong to Link. Be careful when using them, because it's not easy to obtain Fairy Tears. Link has to complete the Cave of Ordeals all over again to obtain another bottle of them.
This is not fun, because just like many parts of Twilight Princess, the Cave of Ordeals is excessively long. At this point, most players see that this is no easy task. It's best to save these for battling Ganon.