Demyx is fought as a boss in Kingdom Hearts II and Kingdom Hearts Union χ. He attacks Sora and his allies in the Underworld, as well as in Hollow Bastion during the Battle of the 1000 Heartless. He can be fought again in a data rematch in Kingdom Hearts II Final Mix.
Kingdom Hearts II
During the first battle, Demyx does not physically attack. Instead, he summons 100 water forms that must be defeated within eighty seconds. Water forms have two shapes, quaver musical notes and Demyx doppelgangers. Striking the Demyx-shaped forms will cause them to transform into note-shaped ones. The note forms can be grabbed with the Reaction Command Wild Dance, which causes damage to many forms over a large area. It is recommended to use the Wild Dance Reaction Command as well as the Fire spell as many times as possible. It should be noted that the note-shaped forms will attack if left untouched for some time. Ethers should be stocked to refill the MP Gauge after a consecutive round of Fire spells, but HP should not be a problem, as defeating the water forms causes HP Balls to drop.
When the fight begins, Demyx summons his "water forms", and Sora must destroy fifty forms in forty seconds. In addition to the Wild Dance Reaction Command, Limit commands also help to destroy the water forms quickly.
After the forms are destroyed, Demyx begins his normal attacks. He will leap across the arena with spouts of water erupting from the ground in his wake, and summon pillars of water around him. To stop him, try to hit him with a combo finisher before he leaps again. Demyx can leap and summon water even when knocked into the air by a combo, so be careful attacking him in the air. Demyx will also summon a rain of bubbles from above, and fire a bombardment of bubbles at Sora directly. He can also summon several spheres of water around himself, which explode after a short delay. Demyx will often summon two or three note-shaped water forms as back-up during the battle, and while there is no timer, these water forms will attack Sora and can cause heavy damage, although Sora can use the reaction command to destroy them and cause collateral damage to Demyx.
Sora may occasionally get the chance to use the Show Stealer command—signaled by Demyx marching toward Sora while playing his sitar and quipping "Ain't it a blast?"—which knocks Demyx into the air and stuns him for a short time. This can backfire, however, as attacking Demyx without triggering the Show Stealer will result in Demyx automatically counterattacking with his sitar and doing major damage.
When he has lost approximately half his health, Demyx will summon ten more water forms that must be defeated in ten seconds, fifteen seconds in the Final Mix version. Afterward, Demyx resumes his normal attacks but also gains a new technique, Water Finish. He will walk towards Sora, swinging his sitar, summoning walls of water in front of him and behind Sora. If Sora uses Guard against Demyx's sitar, he can sometimes avoid being damaged by the pillars, allowing him to counterattack.
Most of Demyx's attacks knock Sora into the air, so Aerial Recovery is very useful. It is highly advised to stay away from Demyx until he is open for an attack. For this reason, Thunder helps greatly, and Wisdom Form is also useful.
An alternate method is to beat all the water forms in the beginning and immediately use Knocksmash. The "Duo Raid" portion knocks Demyx into the air and prevents him from using any attacks. The "Cosmo Boost" portion deals more damage as Goofy flies around hitting him. This combo takes his health down almost completely for an extremely easy win, without using any other magic or Drive forms. However, this method becomes much less effective in Final Mix, as the damage done by Cosmo Boost has been drastically reduced.
Demyx fights in a similar manner to the previous battle, but this time he makes use of more water forms. There are four rounds of water forms at the start, the amount and time limit randomizing among them. The most difficult of the four requires Sora to destroy seventy-five forms in thirty seconds. If Sora physically attacks any of the forms, instead of transforming into a note, it will instead retaliate. What is not immediately known to the player is that a water form hit with Blizzaga now triggers the change into note form, therefore allowing "Wild Dance" to be performed. When reduced to his last health bar, Demyx once again sends out the forms in rounds. The very last of the round is the most difficult, requiring Sora to beat ninety-nine forms in only thirty seconds. Also, Demyx's fountains increase in number and power and some briefly follow Sora for a longer period of time, which somewhat resembles Vexen's Ice Needles Sleight.
It is advisable to stock up on Ethers/Elixirs or any MP-restoring items in the item slots, including Donald and Goofy's, and have Donald and Goofy carry out their attacks very frequently. Disable any Auto Limit support abilities, as this might prevent usage of the Reaction Command "Wild Dance" and might force one into a Limit instead, making it more or less difficult to destroy all the forms in time. The note-shaped water forms do not appear to own their own, forcing the player to have to cast Blizzaga if they wish to use "Wild Dance". Firaga will also instantly destroy the water forms. Due to the number of individual projectiles, entering Limit Form and using Ragnarok can also prove useful.
Alternatively, switch to Wisdom Form and use magic-based attacks. Reflect and Fire will come in handy—as while in Wisdom Form, casting magic is done at a faster rate and Sora can move around quicker.
Also, be sure to reset the Anti-Points counter to zero before the battle, so as to help prevent Sora from entering Anti Form. However, one attack from Anti Form can destroy a form due to Anti Form dealing Neutral damage as opposed to Physical. While capable of defeating the water forms within the time limits, a method revolving around the use of Anti Form requires a lot of skill and some luck, making it less consistently reliable than simply using Wisdom Form.
Heal when necessary, but prioritize MP for attacks; Only use Cure if Donald is KO'd and Sora absolutely must heal.
Notes and references