Disclaimer n°1: Don't play this build. It probably is bad. I'm not a content creator, nor a build maker... There are a plethora of great builds out there made by reliable people who know what they are doing.

Disclaimer n°2: I'm shit at leveling so don't expect any advice on leveling from me ... maybe go pconc following tytykiller's advice and then heist a bit to respect. I'm probably gonna go blazing salvo asap (act 3 siosa) and take 8 to 12 hours to finish my leveling.
If you want tyty's video: https://www.youtube.com/watch?v=6OpL7Kgawq0

TLDR

A pretty fun self-casting playstyle with reasonable tankyness and a very high damage ceiling. Your end goal is building omni and going Raider using forbidden flame/flesh for the mark efficiency node. Its main focus is bossing but once more don't play this build if you're looking for a great build. It's good, probably not great.

After posting this, I'll try to see if I can assemble end-game gear on Standard and see how it feels then.

The theory

Not so long ago, someone posted this: https://www.reddit.com/r/PathOfExileBuilds/comments/w84bab/318_raider_coc_omni_blazing_salvo_1b_dps_cyclone/

The idea is simple, blazing salvo can shotgun, sniper's mark makes projectiles split, so with sniper's mark, your blazing salvo will shotgun more. Now if we add mark effect to the mix, your blazing salvo will shotgun EVEN MORE for huge damage. But... that build has GG gear with tons of specific gear and stats required. So what if we could make a self cast league start friendly version using the same mechanics?

Here are the sniper's mark calculations with blazing salvo and my video testing the theory

Base for sniper's mark is 1 proj becoming 5 projectiles from the split. If you multiply 5 by your curse effect, you will get a number.

At lvl 20, blazing salvo fires 6 additional projectiles (7 total), at level 21 it fires 7 (8 total), and at level 28, it fires 8 (9 total). Let's assume, for league start, that we manage to get our gem between 21 and 28. Let's also assume we have sufficient area of effect and we position correctly (so basically ideal scenario) so that they all hit.

In my video proof - https://youtu.be/cgIZWD_pBo4

- I'm a Deadeye, 75% (ascendancy) + 25% (mastery) + 10% (tree) : 5*(1+0.75+0.25+0.1) = 10.5 (and since poe rounds down that makes 10 proj)

- I'm getting 8 projectiles without sniper's mark but with it, I get 18 which is a 10 projectile difference so I think the math is good.

With my trees

1. League start & red map build

• Deadeye: focal point + endless munitions
• Curse effect: 30% enchant + 75% ascendancy + 25% mastery = 130% ---- 5*2.3 = 11.5 projectiles, rounded down to 11 projectiles from sniper's mark
• Total projectiles: 8 skill gem + 2 endless munitions + 11 sniper's mark --- 21 total projectiles

1. Min-maxed versions

• Raider: focal point (forbidden flesh/flame)
• Curse effect: 30% enchant + 75% ascendancy + 25% mastery + 10% tree = 140% ---- 5*2.4 = 12 projectiles
• Total projectiles: 9 skill gem + 12 sniper's mark ---- 21 projectiles

The showcase

https://youtu.be/V15cB1oejOk

It takes me roughly 10 minutes to clear the map and 2 minutes to kill Veritania.

I die once, to poison balls because I'm not paying attention. I bricked my first run because I wasn't dodging Vertania's spells. Nope, don't do that with this build.

I Would definitely recommend having better gear before running this type of content, this is really to show what the build looks like and feels like on low budget.

The pob

https://pastebin.com/pTTDPBn5

You will find 4 different settings in this pob.

I. League start

• The character is level 90, the gear is suboptimal. It's basically stuff I found in standard to test run the build. The idea is to simulate a character at league start.
• We are running 3 auras: grace, herald of ash, and a low-level clarity to get some mana regen. ^(To be honest, I think it would be better to run vitality since we're using an endless mana flask anyways.)
• We are using Arcanist Brand to apply Sniper's mark and Flammability as well as cast flame wall.
• We are using Wave of Conviction linked to Combustion, the new gem Overcharge and Spell Totem to apply fire exposure, combustion and (hopefully) shock
• We are using Flame dash linked to Arcane Surge
• We have a Vaal Righteous Fire for burst damage
• We are using a Cast when Damage Taken + Immortal Call set-up
• We are using Cast on Death + Portal because Softcore build
• The tree has no cluster jewels and is fairly straightforward.

1. Defense: 4.4k HP ; 1k ES ; Ghostshroud ; 20k Evasion ; 75% Spell Suppression from the tree (before the buffs)
2. Offense: 2.4 million dps in an ideal scenario

The Veritania video is done with this set-up with 2 differences: I have a stygian vise in the video (with the exact same stats and the one from the pob) but don't have the helm enchant.

II. Red maps

• The character is level 93. We have a pretty nice +2 wand and +2 amulet, a good +1 shield. We are using a Crown of the Inward Eye and a Replica conqueror's Efficiency. The rest of the gear is life + base defenses, with no suffixes. This is what I'm aiming to have when I start getting into t14+ content. Please note that there will be some fiddling needed with this set-up for res and stats.
• We are running 3 auras: Grace, Herald of Ash and Zealotry. Depending on your mana, you could run Vitality.
• We are using Arcanist Brand to apply Sniper's mark and Flammability as well as cast flame wall.
• We are using Wave of Conviction linked to Combustion, the new gem Overcharge and Spell Totem to apply fire exposure, combustion and (hopefully) shock
• We are using Vaal Righteous Fire linked to Increased Duration for burst damage
• We are using an Immortal Call linked to Increased Duration to use on left click
• We are using Cast on Death + Portal because Softcore build
• We are using a spell damage cluster for Arcane Heroism (to get Arcane Surge), Essence Rush and 2 jewel spots.
• The rest of the tree is fairly similar to the previous one.

1. Defense: 5k HP ; 1.5k ES ; Ghostshroud ; 20k Evasion ; 69% Spell Suppression from the tree (before the buffs)
2. Offense: 7.5 million dps in an ideal scenario

III. Min-maxed Glass Canon

No way I'm ever getting this gear, but it's to see the ceiling.

• The character is level 97. We have insane gear all around. We're using Omni with a perfect Black Sun Crest, an insane Circle of Anguish, a Mageblood, Bottled Faith, Forbidden Flame and Flesh to go raider but still have the mark effect and a surprisingly reasonable Watcher's Eye with Vitality mod.
• Bassically I tried to see the best damage version I could make in Pob, won't go into detail. You should just look at the pob if you want more details

1. Defense: none lol
2. Offense: 500 million dps in an ideal scenario

IV. Min-maxed Defensive

No way I'm ever getting this gear either, but it's to see an alternative more tanky version.

• The character is level 97. We have insane gear all around. We're using Omni with a perfect Black Sun Crest, an insane Circle of Anguish, a double corrupted Brass Dome, a Mageblood, Bottled Faith, Forbidden Flame and Flesh to go raider but still have the mark effect and a surprisingly reasonable Watcher's Eye with Vitality mod.
• Bassically I tried to see the best damage version I could make in Pob, won't go into detail. You should just look at the pob if you want more details

1. Defense: not much to be honest. We have a Brass Dome but eHP is caca.
2. Offense: 360 million dps in an ideal scenario

Edit: showcase is up

Edit n°2: 2nd showcase, less cringe, comparing raider and deadeye, with better gear is up. Find it here https://youtu.be/AbjFH17xC3Y

I've also had some additional thoughts that would need to be tested after league start.

1. I'm thinking of swapping in rolling magma instead of blazing salvo for clear and pathing to endless munitions with ricochet. If it works how I think it does, it should make clearing much much easier.
2. With some rmr, I'm thinking the new arctic Armour seems pretty interesting for the build since we're always standing still while standing.
3. For the same reasons, the updated Ralakesh's impatience could nice.
4. Kintsugi could be a great defensive option since we're scaling evasion, the problem is that we'd need 6 off colour sockets... Will need to see how easy those are to acquire with harvest changes.
5. Minimum endurance charges grand spectrum would also be a nice boost for survivability against phys damage (and easy ele res). This could be comboed with ele damage, crit chance or minimum frenzy spectrums. If it's worth or not would be depending on the market really