Level 92 Crit Arc Infernalist^0.2

Notes

Or whatever elemental skill you like. 
There's nothing specific for Arc/Spark except for the Prism of Belief jewel and +skill levels (right element) on the wand and Sunsplinter.
I was just messing around and thought I might share it. Maybe it's interesting for anyone that started with Witch/Infernalist and 
doesn't want to level another character to maps. Below you can find a detailed write-up of the concept/my thoughts.
If there are any questions, feel free to ask! I'm currently Lv 94 (~40 deaths) and still testing some things and trying to improve, so I'll update the PoB now and then.
Note, that it's primarily made for fast mapping. Due to its low life pool it'll not be good at pinnacle bossing (I mean you could but won't have a fun time).
Also: This is not a starter build. You need levels and also a little bit of game knowledge (mechanics/boss attacks), therefore it's not super new-player-friendly. 
It also takes some time to get used to the playstyle of life-loss and a lower life pool than usual. 
As for the cost, I guess you'd need about 40 div for good gear and a +2 Prism jewel to make this feel comfortable assuming you want to play T15+ maps 
(prices of course depending on the market/current meta/what's popular or what skill you use). 
If you're looking for a build where you can put money in, here you go. Every upgrade is a big QoL.

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 especially at bosses (life recovery from flask is so high it almost
functions as instant recovery).
As energy shield gets burned and I personally don't like armour (and don't have the strength requirements), the only base defence layer that's left is evasion
(but of course isn't the only defence layer).
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. 
I'd say, the builds biggest weakness are (big) phys hits (mostly from bosses) due to the low life pool but most of the time it's not a big deal because of the 
phys taken as conversion and also decent evade chance and some block chance.

Why Chaos Damage?
Enemies have lower base chaos res than ele res, no need for res pen. Double damage against energy shield.
Can use Wither which means the enemy takes up to 70% increased chaos damage as long as they're in our presence (though it takes a few seconds to reach 
max stacks).
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 Despair & Sigil of Power).
Downsides of this are that you can't inflict ailments (shock/electrocute), can't take Lightning Rod for lucky hits and Arc also doesn't release its damaging pulses.

Defences:
~60% evasion with just one tree passive (up to 65-70% against blinded monsters), 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.
Due to life regen and high res, poison/bleeding/ignite can mostly be ignored.
Hinder, Freeze, Blind against single targets. 
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.

If you want to invest even more into evasion, you have the following options:
- use a 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. This gives you ~100% increased evasion rating. Important: This will almost halve your mana pool which means you reach 
  max Infernal Flame more often and take damage from it.
- there also is a changed timeless notable that gives you 40% evasion + 10% res
- can get up to 50% evasion on amulet 
- can get 44% increase for 3 points on the tree (Inner Faith)
- can get 125% increased evasion on shield for 2 points on the tree (Core of the Guardian)

Another tech is by using a Dousing or Ruby Charm with the 'life on use' modifier. Everytime you get ignited (by Grinning Immolation) or take
fire damage (always), 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

If you use a Timeless Jewel, you have to respec Controlled Magic as this node will get changed. You can take either Careful Assassin, Sudden Escalation
or Critical Exploit instead.

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 (which is an extra 50-100k).

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. 
If you struggle with Spirit on gear, you meanwhile can drop Warm Blooded and Cannibalism. Use a Thawing Charm to get freeze immunity, also get another jewel 
with %life on kill.

Skill Tree:
The tree is set for Level 92, I think this is easily reachable.
If you have enough attributes, you also can take the 3 small spell damage nodes instead of the attributes right at the witch start.
At level 93 you can respec the crit chance at the Controlled Magic cluster and take the two crit bonus nodes.

Good options for further levels are: 
- Any small crit bonus nodes (this gives you 25% crit bonus for Arc and 29% for Spark)
- Small max lightning res nodes
- Small life regeneration nodes
- Spaghettification Cluster (if you're fine with more life loss)
- Changed nodes in jewel radius
- Jagged Shards (1 point)
- Potent Incantation (4 points)
- Core of the Guardian (2 points) - if you have a high evasion shield
- Inner Faith (3 points)

Gems:
Arc for single target/bossing (as it has a very high max hit). 
Use the gem setup listed under skills. Can also use Unleash + Extraction for bigger burst hits but I personally didn't like that.

Spark for easy mapping/clear (also see gem setup under skills). 
When using Arcane Tempo for Spark, you meanwhile put Lightning Mastery in the Arc setup.
You have a few options here: 
- Inspiration: to lower cost and thus the self-damaging frequency (if you do, I'd go with a Level 9 Despair instead of 20)
- Mobility: if you want to go faster
- Overpower: as a more defensive option for more stuns
- Pierce can be used instead of Gambleshot (take whatever you prefer)
- Persistence: for more projectile duration (but is already used in Withering Presence for bossing)

Despair: 
I'm using a setup with Cursed Ground. I think curse zones are pretty nice for mechanics like ritual, strongboxes or at the start of a breach. You also can
pre-cast it on bosses. If you want, you can run Focused Curse over Cursed Ground but I personally don't like the idea of curse delay 
and that you have to use a support gem (with cost multiplier) to reduce it. Could also take Expanse and use Magnified Effect for Sigil of Power.

Additional Information/Tips:
- For better clearspeed, you can get a second Prism of Belief with +Levels to Spark.
- Rather use a +4 wand with high spell/chaos damage than a +5 with low stats.
- Make sure you buy an Original Sin with 100% conversion (there's some corrupted ones with less than 100%)
- 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.
- This is also true for "extra damage gained as (elemental)" modifiers. They're calculated off the unmodified base value and don't seem to be 
  converted to chaos damage, so there's actually no scaling (skip this mod on wands).
- 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.

Expensive Gear Upgrades:
+5 Skill Wand (with high damage, cast speed, crit)
Better amulet (base type/attributes)
Corruption on amulet/weapon/jewels
2 socket boots 
Megalomaniac with 3 stats
+4 Skill/max fire res Sunsplinter