The Cold Shower (Crit-Cascade) [1.3]

Strengths:
Cheap. Low-budget, but scales well with gear (e.g. 6-link and crit)
High effective life
Freeze, shock, and shatter
Mind over Matter. Empowered Arctic Armor
Block/Spell-block
Solo high level content
Hardcore viable options.
Deeps! (while also staying defensively strong.)

Weaknesses:
AoE is awkward at first. Clear speed is slow with little increase to AoE.
Too much AoE is bad, but only at really high amounts.
STR and DEX requirements are hard to meet (mostly DEX)
No-regen and Blood Magic maps are hard.
Reflect maps are sketchy, but reflect rares are not bad with a Sapphire flask.

For those new to Glacial Cascade, this spell doesn't hit just once per time it's cast. The spike-waves come out of the ground, move forward and overlap, dealing cold and physical damage. Per one cast you can expect to hit a target at least two or three times. More AoE equals increased wave size but also bigger gaps in the waves (i.e. fewer hits.) For best results, try to catch most things in a mid-range of the AoE. Sometimes GC may hit only once if the enemy is on top of you or too far away.

I'm very sure that the tooltip number represents the DPS if you were to hit exactly once per cast, or in other words, doing the bare minimum damage. The actual average DPS is the tooltip number multiplied by 3. Similar to shotgunning with LMP/GMP spells.

Glacial Cascade has a 80% damage effectiveness penalty.

Damage modifiers:
Generic spell damage.
Critical Strike Chance. Important until you reach 50-60% chance with charges, then it gets better to stack Crit Multiplier.
Cast speed.

Cold or elemental damage doesn't scale efficiently because the mixed damage types.

Herald of Ice Flat cold damage with little mana reserved. Plus GC freezes and shatters a lot.

Haste or Hatred? Personal choice, Haste is better damage. You just lose a lot of defense in favour of DPS (although movespeed is a form of defense itself. Vaal Haste is great for burst damage. Haste/Vaal Haste have the highest dex requirements which can be a pain in the ass. You can usually get these auras from playing with a party.

Herald of Thunder Flat lighting damage, and gives GC ability to shock. Plus auto-curse

Purity of Ice Option for elemental reflect.

Discipline? Only use if you can't sustain AA without it. I don't think a larger mana pool is worth having less unreserved mana to soak up damage with.

I don't have many tips to put here. Just go for important passives first and fill in the rest as you can. Use Freeze Pulse/Ice Nova or another skill until you can get GC from the quest reward from killing Brutus in Normal.
A Lifesprig is good enough to get to Merciless.
You can start maps on a 4-link.

The only problem I've ran into when gearing is dex requirements. Fortunately, lots of slots can roll additional dex or have it crafted on. Life and resists are the biggest priorities, followed by anything to boost mana/mana-regen.
ES on items actually gives more for mana than +max mana. This is because it still gets %increased energy shield before it converts and then gets %increased mana. You can pretty good gear for cheap on poe.xyz if you shop around. Good crit gear will generally be more expensive. Budget items work but the minimum is a Cloak of Defiance.

Uniques vs. Rares
Besides Cloak of Defiance and spell-block items, you're better off with solid rares.
The Doryani's cold belt gives some leech, but a rare belt is better for life and resists.
The efficiency of Rathpith for life varies depending on your other gear, but spell block and spell damage is always nice. Rathpith also has synergy with Rumi's flask.
Divinarius is a good endgame weapon that isn't really expensive. Stronger rares are hard to find for cheaper. Maligaro's are awesome for the crit, but lacks life.

Flasks
Try to get "Surgeon's" on flasks where you can. Curse immunity on one flask.
One or two life flasks; at least one that removes bleeding, and instant pots are recomended. Granite/Jade flask with % increased armour/evasion, eventually get a Rumi's as an upgrade. A Sapphire flask (new unique one is pretty cool.) Then for the last slot use Diamond or Atziri's Promise (what I use,) or whatever. Atziri's promise is nice because it's hugely helpful against chaos degen stuff, and destroying things while leeching a lot.
See gear section for my flasks.

Bandit Choices: Life - Passive Point - Power Charge

The support gems on Glacial Cascade will depend on whether or Vaal Pact is used. If not using VP, then use the same links but change life leech to either Increased Crit Damage or Added Chaos.
4-link: Glacial Cascade - Spell Echo - Faster Casting - Life Leech
5-link: - Increased AoE (swap for Conc. effect on single-target)
6-link: GC - Echo - Faster Cast - Life Leech - Conc. Effect - Increased AoE (swap for Inc. Crit Damage or Added Chaos on single-target)

Concentrated effect: the best damage support, but it's not something to clear maps with unless its downside is countered. Using Inc AoE support gem is one way, or it can be compensated for elsewhere with heavy investment (gear and tree-wise.)
Spell Echo: hard to use without Faster Casting, because it will make you pinned down for too long.
PCoC: only useful in certain situations.
Inc. Crit Damage: good for bosses and gives a high damage potential.
Added Chaos: consistent damage, and is the strongest "added X" support gem.
Empower: not optimal if it's only level 3. I don't have a level 4 Empower or a +1 chest to test with, but it still may not be stronger than the other, cheaper, damage gem options.
Cold to fire: a support I see in other people's builds, but I don't think it's best because the physical and lighting damage doesn't benefit.

Clarity - Herald of Ice - Purity of Ice (or Haste) - Reduced Mana

Arctic Armour - Empower - Enhance (both are not necessary supports)

Assassin's Mark - Curse on Hit - Herald of Thunder - Reduced Mana
This setup is removes the need to manually apply curses. You just crit and kill a mob first to get it going, and it keeps your power charges up.

Lighting Warp - Faster Casting - Reduced Duration
Used to get around and escaping. Surprisingly strong when it crits.

CwDT setups; Use one or the other, or neither. Not enough slots for both though
Cast When Damage Taken (lvl 1) - Frost Wall (lvl 5) - Enduring Cry (lvl 5)
Frost wall gives pushes stuff into the sweet spot for GC (Frost Wall doesn't stop GC) and it is a helpful defensive utility. EC is extra physical mitigation.

Cast When Damage Taken - Enduring Cry - Immortal Call
Gem levels don't matter as long as the player level requirements don't surpass CwDT. Enduring Cry should be closest to CwDT so it triggers first.

Utility setups to play around with:
Cold Snap - Trap - Elemental Proliferation (- Cold Penetration)
Handy for strongboxes. I don't use it anymore because I have no extra room for the gems. Also replace Cold Snap for Ice Nova, or Vaal versions of either one.

Warlord's Mark
Leech.

Enfeeble

Rejuvenating/Devouring/Decoy Totem
The regen from a Rejuv. totem doubles my life Witch's life regen.

These are numbers spit out by the game. The spell hits 3 times on average per cast, and then there's Assassin's Mark when using it.
On crit I shock on the first hit, and the immediate hits get +50% damage. And since the crit is rolled once for the entire first and echoed casts, all of those spikes will crit as well, not only reapplying freeze and shock but making them longer (duration being a function of damage vs targets max life). This only works if you inflict a minimum shock duration, which may seem hard with just HoT lighting damage, but with crit multi it's no problem. Added lighting can be used to help reach this minimum amount.
Not accounting for curses, shock, or mob/boss resists, the DPS to anything in range is 3 x 46k = 138k

Reflect:
It's manageable without VP but it's easier to one-shot yourself. Use a Sapphire flask for rares, and run Purity of Ice plus turn off Herald of Thunder or just change Assassin's Mark to Warlord's mark (easier to just take out curse on hit, and apply Warlord's Mark socketed in an unset ring.)
Vaal Pact:
this is personal and will change playstyle a bit. I'm trying it out, but how good it is really scales with leech. So you want to use a leech gem (at the cost of a support slot), and maybe curse on hit Warlord's Mark, Berek's, or cold leech. Point investment is not huge but still means losing something else.

http://www.pathofexile.com/passive-skill-tree/AAAAAgYADXwRDxSwFr8XVBzcIoEmlScvKPospi1HNAo1kjdmN4M6WDrhPC1Bh0V8RZ1KyEwtTeNQQlNSVUtVxlZKV8lfKl8_YeJnm26qdFV343_Ggh6Cx4qvjDaOPJARkyeXl5rgnqGgtKKjpKymV6Z_puusmK4-sbO0DLQ4tUi18rc-vJ_B88au2L3jn-RR5Rnquuvk6-7tQe8O8kXzBvno-6r79f4K_o8=