Elemental Damage

In the later stages of the game, the damage done and received by enemies becomes integral to the ability of defeating even regular monsters. The monster list contains info on enemy elemental attributes, but here's a quick list of what you can use to cause elemental damage, and how you can protect yourself from it.

Remember that many monsters also use the Spells listed here; check the Monster list.

Fire:
Spells: Fireball, Eruption, Fire Wall, Flare Tornado, Explode, Fire Storm
Summon: Efreet
Techniques: Phoenix, Magma Rift, Firebrand, any combos with those techs
Weapons: Knight's Sabre, Flamberge, Flaming Sword, Flare Spread (Bow)

Water:
Spells: Ice Needle, Ice Tornado, Ice Wall, Tidal Wave
Summon: Undine
Weapons: Tidal Axe, Ice Coffin, Ice Scimitar, Vorpal, Wave Spread (Bow)

Thunder:
Spells: Lightning, Thunder Blade, Indignation
Summon: Volt
Techniques: Soul Wave (first part), Lightning Bolt, any combos with those techs
Weapons: Thunderclap, Crescent Bow

Earth:
Spells: Grave, Stone Wall
Summon: Gnome
Techniques: Psion Bolt, any combos with that tech
Weapons: Dragon Tooth, Duelist's Sword, Muramasa, Slayer Sword, Sand Spread (Bow)

Wind:
Spells: Storm, God Breath
Summon: Sylphs
Weapons: Arc Wind, Gale Spread (Bow), Grimorum (Book), Sylph Broom

Holy:
Spells: Ray
Summon: Luna, Aska
Weapons: Saint's Rapier, Holy Sword, Moon Falux, Laser Blade, Holy Rod

Dark:
Summon: Shadow
Weapons: Demon Hand, Bahamut's Tail, Excalibur, Spinesheath, Soul Stealer (Bow), Rune Rod

Note that it's useless to have more than one item which completely absorbs elemental effects. The FLAME and AQUA capes, and the SILVER hood, only offers 30% protection from their elementals; the Princess Cape hood offers 50% protection; and the Nymph's Ring gives 100% protection (no damage). All other armors will absorb the elemental as HP.

Fire:
Dark Robe (Arche), Golden Armor (Cless), Gaia Armor (Cless/Chester), Rare Plate (Cless/Chester/Klarth), Garnet Ring (Klarth), Silver Cape/Princess Cape (Mint/Arche), Flame Mantle (all), Nymph's Ring (all)

Water:
Dark Robe (Arche), Gaia Armor (Cless/Chester), Reflect Plate (Cless/Chester), Star Cloak (Mint/Arche/Klarth), Rare Shield (Cless), Aquamarine Ring (Klarth)

Thunder:
Dark Robe (Arche), Golden Armor (Cless), Holy Cloak (Mint/Klarth), Gaia Armor (Cless/Chester) Reflect Plate (Cless/Chester), Rare Helm (Cless), Sapphire Ring (Klarth)

Earth:
Dark Robe (Arche), Rare Gauntlet (Cless/Chester), Amethyst Ring (Klarth), Flame Mantle (all)

Wind:
Dark Robe (Arche), Golden Armor (Cless), Reflect Plate (Cless/Chester), Sardonyx Ring (Klarth), Aqua Mantle (all)

Holy:
Gaia Armor (Cless/Chester), Mithril Plate (Cless/Chester), Mithril Mesh (all), Moonstone Ring (Klarth)

Dark:
Golden Armor (Cless), Gaia Armor (Cless/Chester), Reflect Plate (Cless/Chester), Star Cloak (Klarth/Mint/Arche)