Buy many Resurrects, Healing Drinks, Antidotes, Restoratives and Mage's Souls. Have your party at least at level 30. Take a person with a lot of damage in your party, and equip the best equipment there is, as follows:

Equipment for Kite or Long Arm User:
Weapon: Dante's Blades or Hell's Gate (Twin Blade), Fiend Spear (Long Arm)
Helmet: Fire Dance Hat
Armor: Firedrake Mail
Hand Armor: Miner's Gloves or Smith's Gloves
Leg Armor: Fire Lizard
Equipment for BlackRose, Blademaster, Heavy Blade, or Heavy Axe:
Weapon: Claymore (Heavy Blade), Jinsaran (Blademaster, Secret Rare Blademaster Sword) or Sun Fang One or Kikujyumonji, Devil's Axe (Heavy Axe)
Helmet: Hunter's Hood or Forester Helm
Armor: Blaze Armor
Hand Armor: Hands of Fire
Leg Armor: Electric Guard
Equipment for Mistral or Wavemaster:
Weapon: Starstorm Wand
Helmet: Scarab Earring
Armor: Lincoln Green (From Cima)
Hand Armor: Air Bracer
Leg Armor: Oak Anklet

To defeat Skeith, distribute twenty Resurrects to each person in the party, then thirty Healing Drinks and twenty of Antidote and Restoratives. Do not distribute the Mage's Souls. Use the entire group at first and use skills. If Kite dies, press Square for "First Aid" and afterwards, just keep hitting him without skills. Once you get enough for data hacking, use it on Skeith and you will see his second form. Because of this, just do a hit and heal combo so you will not get major damage from the shockwave hits. Keep repeating this combo so you will not die. And if you do die, keep using "First Aid" and keep slashing away.