Dark Valefor uses physical attacks and Sonic Wings, which inflict Delay (weak). Dark Valefor can also use Energy Ray at any time. Unlike the summon Valefor's Energy Ray, Dark Valefor's Energy Ray is considered magical damage and can be reduced by a high Magic Defense stat or other protections against magic. Dark Valefor's Overdrive is Energy Blast, which deals around 50,000 damage regardless of stats. Unlike the other Dark Aeons, Dark Valefor cannot break the 9999 damage limit except with Energy Blast, which always deals 47,136 to 53,225 damage, and cannot be reduced with anything but another aeon'sShield.
It is best to use Auto-Life, or summon an aeon when Dark Valefor's Overdrive gauge is full to take the blow.
Dark Valefor can be defeated quickly, before she causes trouble. The party should have high Agility and Luck. High Strength and Defense are also useful, but all stats should be well into three digits. Break Damage Limit is recommended; Dark Valefor is also perhaps the one enemy where Break HP Limit provides more benefits than drawbacks due to her inability to exceed 9999 damage. Valefor's Overdrive gauge charges up quickly, so the party needs to be fast.
Sometimes in the International PS2 version, a glitch will occur and the game will crash on either the cutscene before or after the fight. If this glitch happens once, it will always happen in that save file, effectively preventing the player from ever fighting Penance, acquiring the Jecht's Sphere in Besaid Village, acquiring Valefor's Energy Blast Overdrive if not obtained on the first visit, and even from acquiring Anima (and subsequently the Magus Sisters) if the player did not get the temple treasure from Besaid at the beginning of the game. To bypass this glitch, one must simply open the PS2 CD tray while the scene that is supposed to freeze starts (the scene will stop while the tray is open) and then close it. The saved game editor FFXED is also capable of fixing a glitched save file.
The Greatest Hits version partially fixes the glitch. A new game started on this version will never have the glitch; however, the glitch will still occur when loading an already-glitched save. The PAL version fixes the glitch completely, even when loading a glitched save from the International version. By extension, this also means the glitch is fixed in the HD Remaster.
After the battle with Dark Valefor, the game reuses two lines of dialog of two non-player characters from the fireplace scene in Besaid Village. The reused lines are "Stay away from the summoner!" and "You're a bad man!"