The Lightning Set consists of all Items that give protection against Energy Damage. Wearing this full Set will give you 43% energy protection, but increase Earth Damage by 34%.

Sorcerers can replace the Lightning Robe with Velvet Mantle and obtain 47% Energy resistance and 38% Earth weakness.You can use Might Ring to earn 20% of protection against Energy, obtaining 67% protection against Energy.

Sorcerers and Druids can replace the Lightning Robe with Spellweaver's Robe and obtain 45% Energy resistance and 36% Earth weakness.You can use Might Ring to earn 20% of protection against Energy, obtaining 65% protection against Energy.

Notes:
You can use Strange Talisman instead of Lightning Pendant to get 33% protection against Energy, increasing 24% Earth Damage.
You can use Shockwave Amulet instead of Lightning Pendant to get 63% protection against Energy, increasing 24% Earth Damage.
If you use Shockwave Amulet and Might Ring you will get 83% protection against Energy, increasing 4% Earth Damage.

Item	Attributes	Dropped by	Level
Lightning Pendant[1]	energy +20%, earth -10%	Diamond Servant, Massive Energy Elemental, Wyrm, Yielothax.	60
Lightning Headband	energy +4%, earth -5%	Giant Spider.
Lightning Robe	energy +8%, earth -8%	Warlock, Zarabustor.	50
Lightning Legs	energy +6%, earth -6%	Black Knight, Massive Energy Elemental, Yielothax, Ferumbras, Zarabustor.	40
Lightning Boots	energy +5%, earth -5%	War Golem, Gravelord Oshuran.	35
Also of interest
Strange Talisman[1]	energy +10%	Elder Mummy, Mummy, Mutated Human.
Spellweaver's Robe	energy +10%, earth -10%	Draken Spellweaver, Ghastly Dragon, Undead Dragon, Hatebreeder.	60
Velvet Mantle	energy +12%, earth -12%	Ferumbras.	75
Shockwave Amulet	physical +60%, energy +40%	Diamond Servant, Massive Energy Elemental, Wyrm, Yielothax.	80
Might Ring[1]	physical +20%, fire +20%, earth +20%, energy +20%, ice +20%, holy +20%, death +20%.	Ashmunrah, Demon, Diamond Servant, Hero, Worker Golem, Yielothax, Latrivan, Morgaroth, Zugurosh.

1. ^ ^{1.0 1.1 1.2} Lightning Pendant, Strange Talisman, and Might Ring have limited charges.