Level 92 Crit Arc Infernalist^0.2

Notes

Or whatever elemental skill you like. There's nothing specific for Arc/Spark except for Prism of Belief jewel and +skill levels on the wand and Sunsplinter.
I was just messing around and thought I might share my build. 
Maybe it's interesting for anyone that started with Witch/Infernalist and doesn't want to level another character to maps.
Below you find a write-up of the concept/my thoughts.

Why Infernalist?
Primarily because you can completely ignore mana, cost and regen (mana is only good for increasing the mana pool which means it takes longer to reach
maximum Infernal Flame and taking damage from it).
We also can take very strong passive nodes like Vulgar Methods, Barbaric Strength, Overzealous and Final Barrage without the downsides.
Can reserve 25% of life for spirit; paired with Coward's Legacy this means we count as low life. This way we can take Pain Attunement without its downside.
In total, we get 50% more crit bonus from Grinning Immolation, 30% from Pain Attunement and 15% from Blindside.
As we need no mana, we could possibly make use of Heroic Tragedy to change the Int bonus from mana to evasion to increase evade chance even further.

Downsides:
We constantly take life and energy shield as fire damage because of Grinning Immolation and Pyromantic Pact. 
This means we need 90% fire res, life regen, life on kill and have to use our life flask often.
As energy shield gets burned and I personally don't like armour (and don't have the strength requirements), the only main defence layer that's left is evasion.
Also, the only defensive node from the ascendancy is damage conversion to fire/chaos.
We have a low life pool as 25% gets reserved and there's no life on the rings, belt, shield and body armour. 

Why Chaos Damage?
Enemies have lower base chaos res than ele res, no need for res pen. 
Can use Wither which means the enemy takes up to 70% increased chaos damage as long as they're in our presence.
Good tree pathing with many damage, crit, cast speed nodes and jewel slots.
No 'combo gameplay', most of the time it's a one-button build (sometimes cast curse & Sigil of Power).

Defences:
~60% evasion (up to 65-70% with blind against single target), 20% block from shield, life regen, life on kill, 90% fire res, 78-82% cold/lightning res, high chaos res.
40-49% of phys taken as fire (body armour corruption), 20% of phys taken as chaos, 20% of cold/lightning taken as fire.
Evasion can be easily achieved and is not conditonal like Wind Dancer or Tailwind. Chance to evade is permanently up at all times while in combat.
Due to life regen and high res, poison/bleeding/ignite can mostly be ignored.
Hinder, Freeze, Blind against single targets. 

You can increase evade chance to ~70% (~75% with blind) by using the Heroic Tragedy jewel and socket it near Chaos Inoculation. This costs you 1 point
(and one rare jewel) but also opens up new passive options as some nodes get changed. 
Important: This will almost halve your mana pool which means you reach max Infernal Flame more often and take damage from it.

Another tech that can be used, is using a Dousing Charm with the 'life on use' modifier. Everytime you get ignited (by Grinning Immolation), you get some life
back from the charm. It's a nice extra recovery for mapping.

Could also consider using Rise of the Phoenix instead of Sunsplinter for a ton of life regen (6-7%).

Lastly, Eternal Youth + Energy Shield gear instead of Evasion could also be an option.
But you'd need a lot of ES on gear + spend passives in ES and increased flask recovery and the tree is already tight on points.
You also need to constantly spam life flask as you take every hit.

Timeless Jewel (if you use):
Heroic Tragedy with Olroth. The number doesn't matter. Can look for one that changes nodes in radius to any other good node (or put points elsewhere).
Good node options I've seen so far (after testing multiple): +2% max res, 1,5% life reg, chaos res, 40% evasion + res, 37% chaos damage, 40% spell damage,
skill effect duration + mark effect

Maths/calc information:
Wither: 7 sec. base duration +45% increased (tree) x 1,4 (persistence) = 14,21 sec / 1,4 (stack/sec) = 10,15 stacks (= max)
Presence: 4m base + 85% (tree) + 40% (jewels) = ~9m radius
Crit Chance: 60% more from Pinpoint, 15% more from Blindside, 20% more from Charge Infusion
Crit Bonus: 30% more from Pain Attunement, 50% more from Grinning Immolation, 15% more from Blindside
Damage from CoC/ED is not included.
20% more damage from Deliberation is added as custom mod.

Requirements:
Attributes: 205 int (for lv 20 gems), >20 str (for gem slots), ~120-140 dex (depends on gear & Sniper's Mark level).
Spirit: min 200. You get to 170-180 automatically depending on your life pool. The rest comes from amulet or helmet corruption. 
Or drop Warm Blooded and use a Thawing Charm to get freeze immunity.

Skill Tree:
Good options (for further levels) are: 
Any small crit bonus nodes, small max lightning res nodes, Spaghettification Cluster (if you're fine with more life loss), changed nodes in jewel radius

If you have enough attributes, you also can take the 3 small spell damage nodes instead of the attributes right at the witch start.

Gems:
Single target/bossing:
See gem setup listed under skills.

Mapping/clear:
See gem setup listed under skills.
You can also use Inspiration to lower cost and thus the self-damaging frequency and Mobility if you want to go faster.
If you use Inspiration, I'd go with a Level 9 Despair instead of 20.
Overpower is a good, more defensive option for more stuns.
Pierce can be used instead of Gambleshot, take whatever you prefer.
Or you can of course use any of the supports from Arc and use for Spark for mapping (I'm just too lazy to swap gems all the time).

Additional Information/Playstyle:
Avoid the following map modifiers: reduced life recovery, -max res, elemental weakness cursed (if you're not res overcapped).
Keep in mind that damage conversion works differently in PoE2. Increased damage to the element does nothing as the original stat is not affected, 
therefore Elemental Focus doesn't work.
Don't forget to set your skills to weapon set 1, so you instantly switch back to the right weapon after using Sigil of Power.
I recommend not using Extraction. It's an insane damage boost but it will shred your life (be aware that you can kill yourself).
Megalomaniac isn't necessarily needed. Another rare jewel is fine aswell. If you use one, you can increase your chanes of finding a good one by also looking
for Escape Strategy and anoint something like Desperate Times on your amulet.
While mapping, you'll have a decent amount of life on kill for extra regen. For bosses, you need to play a bit more careful. Timing is important; wait for the
right moment before you spam Arc.
I also was looking for a cheap way to reach ignite immunity as Cloak of Flame already gives you about 50% reduced ignite duration. But there aren't really
any good options that are worth it. Can't get this stat on helmet/gloves/boots/jewels, no uniques and runes for it and Fan of Flames (tree passive) only gives you 
25% reduced duration. Cool Headed (gem) is multiplicative, not additive. Against the Darkness (Diamond) would be an option but it only has a small radius, so
you'd need a Megalomaniac with Fan of Flames + Against the Darkness which means a lot of damage loss. Anyways, I eventually decided to ignore this because
if we do the maths we take 15% of life+ES per second as fire damage which means at 2k life+ES we take 300 damage which is reduced by 90% fire res. 
That's 30 damage per second and the Stand Ground passive gives you 1% life reg while affected by a damaging-ailment and almost counterbalances this.