Keraunomancer

It must be mentioned with an emphasis that it is, and thus it shall remain, the author's opinion that the raw, cackling powers of nature should not be tempered with at any event. As these powers have unfortunately been discovered, however, they are, one may assume, to be put to use soon or late. We therefore resolved, in an attempt to prevent the greater evil by choosing the lesser, to guide those who would be unable to resist the temptation with the following notes on the mastery of keraunomancy, the arcane arts, that is, of lightning and electrocution. Whether these instructions may be used for good or ill, it is not ours to decide, yet we urgently plead with the reader and student of these writings to take the utmost care they can possibly muster during the procedure of both learning and applying the aforementioned art.
Side effects may, inter alia, include the consumption of sanity by the acquired powers as the result of an ever-growing hunger for destruction and laying waste to entire battlefields within a matter of seconds.

Note:
Since the game is not finished yet, this guide should by all means be considered a work in progress. I am going to update it, as we get to level up further and find legendary items. In addition, I am going to add more detailed information to most of the sections along the way.

Playstyle: Spell Caster
Class: Druid (Shaman + Arcanist)
Specialization: Keraunomancer
Major Damage Types: Lightning Damage + Electrocution Damage
Minor Damage Types: Physical Damage + Cold Damage
Damage Output: High
Survivability: Medium to High
Energy Management: Easy
Suitable for Veteran: Yes
Suitable for Hardcore: Yes
Suitable for Challenge Areas: Yes
Recommended for Advanced Players: Yes
Recommended for New Players: No*

*Note:
This build requires you to suck it up for quite some time. All of the major skills of this build lie pretty far down the road of the respective mastery, so new players will most probably find trying to use this build more frustrating than fun. Apart from that, a lot of kiting (running around the battlefield in order to avoid being hit by enemies) is necessary as well, even though it is not a glass-cannon build.

Physique: +45 (46)
Cunning: 0
Spirit: +15

Distribute the points in a 3:1 ratio.

Example:
Level 2: +1 Physique
Level 3: +1 Physique
Level 4: +1 Physique
Level 5: +1 Spirit
Level 6: +1 Physique
Level 7: +1 Physique
Level 8: +1 Physique
Level 9: +1 Spirit
etc.

Put the additional attribute point from the quest wherever you want, it does not make a difference; I put it into physique.

Why a 3:1 ratio?
First, as a spell caster, you do not get that much health to begin with, so you want to put more points into physique than you would if you were to build a melee character. Spending twelve points in spirit suffices for this build; personally, I have never even once run out of energy, and I have never used an Elixir of Spirit.
Second, this is not intended to be a glass-cannon build, and the damage output is high enough.

Shaman Mastery: +50 (maximum)
Wind Devil: +16 (maximum)
Raging Tempest: +6
Maelstrom: +12 (maximum)
Storm Totem: +16 (maximum)
Storm Caller's Pact: +12 (maximum)

Arcanist Mastery: +15
Trozan's Sky Shard: +16 (maximum)
Inner Focus: +20 (maximum)
Mirror of Ereoctes: +12

The following items are to be understood as suggestions and may be deviated from or improved upon to your own liking.
Notes:
(1) I originally intended to include screenshots from my own game, but there is an issue I have not been able to resolve yet. Thus, links to the corresponding pages on grimdawn.wikia.com will have to suffice for the time being.
(2) For some of the items, no pages yet exist on grimdawn.wikia.com.

Head
Tempest of the Void[grimdawn.wikia.com]

Amulet
Thundering Gem or Stormcaller's Gem[grimdawn.wikia.com]

Rings
(1) Chosen Storm Seal
(2) Chosen Storm Seal or Starfire[grimdawn.wikia.com]

Main Hand
Sacred Hammer of Eternal Wrath[grimdawn.wikia.com]

Off-Hand
Stormcaller's Effigy[grimdawn.wikia.com] or Storm Scion[grimdawn.wikia.com]

Shoulders
Shellshock Epaulets[grimdawn.wikia.com]

Hands
Thundertouch Bracers[grimdawn.wikia.com]

Chest
Skinflayer Raiment[grimdawn.wikia.com] (for the time being for want of a better alternative)

Legs
Templar's Leg Armor[grimdawn.wikia.com] (for the time being for want of a better alternative)

Feet
Desecrater Treads[grimdawn.wikia.com]

Rune
Squall[grimdawn.wikia.com]

Belt
Skyreach Sash[grimdawn.wikia.com]

Medal
Chosen Star of Thunder

December 1st, 2015:

*Iskandra's Elemental Exchange has been replaced by Inner Focus. It actually took me about one thousand hours of gameplay to discover that the former only works with weapons. My apologies.

*Added Mirror of Ereoctes as a new skill. If you get into trouble, it absorbs all damage and reflects 150% of all damage taken. The perfect escape skill.

*Adjusted number of available attribute and skill points to level 60.