[2.4] I Prepared Explosive Arrows This Morning! This is a Buff.

Explosive Arrow is a Fire, Attack, AoE, Duration, Bow skill.
That means it's an Attack, not a spell, with a Bow. Attack Speed affects it, and Attack Speed with Bows.
Duration applies to the Buff EA places on enemies - reduce it's Duration, it goes BOOM sooner.
AoE means it's affected by Area of Effect increases and reduction, and by Area Damage modifiers.

Now, as for damage:
Explosive Arrow's arrows don't do any damage that matters except to trigger Elemental Equilibrium (with Flat Cold/Lightning from gear).
The BOOM is affected by Area/Projectile/Fire damage modifiers, as well as Global Damage modifiers.

For area clear, that's all you really need to know.
For single-target, you can rely on 1 of 2 options:
1) Shotgun Mechanics!
Enemies that stand next to a wall can get hit multiple times each shot if you use GMP. That can drastically increase the damage output.
2) Ignite!
The method I usually use. Almost all the modifiers that apply to the BOOM apply AGAIN to the Ignite, leading to MASSIVE DAMAGE. The way Ignite works, you trigger it when you deal a Critical Hit, or if you have Chance to Ignite - the first is harder to scale on this build, but you have some, the second you have from Quality on the EA gem, and the Chance to Ignite support gem, as well as some Unique items I'll go over later. Being an Elementalist with Conflux means you Ignite reliably without any of those, however.

Fire Resist lowers the damage of EA, and AGAIN on the Ignite, so it's important to lower that. Elemental Equilibrium takes care of that nicely, along with the Fire Penetration gem. Simply make sure your arrows deal Cold or Lightning damage, and you're golden, tap and BOOM and tap again :). Use Flammability if you feel you need even more of this, but be aware this rarely works in parties.

820 * 5 = 4100 avg Base
657% Increased / 697% with The Overflowing Chalice
28% More (7 Frenzy)
40% More (Elemental Overload)
50% More (Shock from Vaal Lightning Trap or Conflux)
88% More (Fire Penetration 21 + Equilibrium)
20% more (Less Duration 21)
30% more (Slower Projectiles 21)
60% more (Concentrated Effect 21)
---
391482 / 412168 Boom!

Ignite:
80% of the Boom Damage over 4 Seconds as Base Damage
657% Increased / 697% with The Overflowing Chalice
28% More (7 Frenzy)
40% More (Elemental Overload)
50% More (Shock from Vaal Lightning Trap or Conflux)
50% More (Equilibrium)
20% more (Less Duration 21)
30% more (Slower Projectiles 21)
60% more (Concentrated Effect 21)
---
23859579 / 26447662 Ignite (over 4 seconds)

Under optimal conditions, no Fire Res, etc, that means around 5+ Mil DPS single-target. Realistically, this is often less, but still more than enough!
I also assumed you don't have a Flammability on, due to party-play.

or (as needed)


Flasks:


Off-hand (Whirling Blades):

Can also one of the other 30% Attack Speed uniques and move just as fast.

Jewels:


Elemental Reflect Mode:


I still need to make one with 40%+ Fire Res to overcap for Ele Weak, atm I'll be missing 9 Fire Res then, but on a Reflect map that's still better than not using the quiver.
Along with the Ascendancy points, you have 50+10+30 = 90% Reduced Elemental Reflect Damage Taken.
Now, you make these using:


Enchant-wise, Flame Golem Effect / Ice Golem Effect / EA Attack Speed / EA AoE are best. EA Damage is terrible, since it doesn't double-dip for Ignite, unlike the Flame Golem.
Boots you want Life Leech, Life Regen, or +1 Frenzy, depending on whether you have Leech from your Ascendancy or not and your damage (0.2% might be enough).
Gloves you want "of Spite" / "of Light" / "of Reflection".

Corrupt-wise, 0.2% Fire Leech or +1 Arrow on Quiver, 0.2% Fire Leech or +1 Frenzy on Amulet.
If you're insanely rich, +1% Max Res on Kaom's Heart, 0.2% Leech or +1 Arrow on Quill Rain. If you somehow get +2 projectiles, you can remove GMP in favour of Empower 4, for example...
If you get a +1 Curse amulet, it's certainly an option for boss fights, using Flammability + Elemental Weakness.

Some people go with another Quill Rain or a +3 bow in the off-hand to avoid swapping gems, and this is quite viable, if expensive. I'd use Empower on a +3 bow instead of Faster Attacks if I did that. There's no need for this, but it's quite possible.

It's quite possible to use a combo of Mokou's Embrace + Eye of Innocence for 2% Leech, Damage, Attack Speed, and 20% Chance to Ignite. As a Witch, that even add 40% Increased Fire Damage on top, due to Paragon of Calamity (from the Ascendancy). I'm not a fan of losing Life on 2 items, but it's quite possible, and not a terrible idea.
It's trading Life on 2 items for 2% Leech, 20% Chance to Ignite, 20% Attack Speed, and maybe a bit of damage (depends on the ring / class).

If you go crit, an amulet and rings with crit are better.
If you go with Acrobatics, you'll want the +% Dodge Gear (Boots/Amulet/Chest, at least some of them).
If you want more move speed, get Devoto's Devotion and lose some defences and Elemental Overload reliability.
If you have enough Attack Speed (Raider, for example), a different Helmet Enchant is better.
If you went with CI, ES gear is best obviously, and you'll be using Soul Strike as a Quiver.
If you can get/make decently rolled Opal Rings, they are BiS for EA.
If you're a Pathfinder, the Dying Sun is BiS, and may well allow you to get rid of GMP in favour of something like Chance to Ignite / Empower / Increased AoE.
Link-wise, if you're not a Witch (or possibly a Pathfinder), you'll not be able to clear well with Concentrated Effect, so you'll only use that for Single-target usually.
If you want to go Full Glass Cannon mode, grab the max Grand Spectrum Jewels and go Berserker, you'll end up with hundreds of millions of DPS and a very different playstyle.

Jewels:
Life/ES + 1/2 Damage mods + Relevant Resistance as needed.
Fire/Area/Projectile/Damage are the best damage mods, the Fire rolls highest, the Damage rolls lowest.
Second tier damage mods are the various Attack Speed ones. You prefer actual damage mods since they double-dip with Ignite.

Bandits are Oak (Life) / Kraytin (8% Attack Speed) or Kill All (Point) / Kraytin (Frenzy)

Very many builds are viable. You can do BM, you can do MoM, you can do Acro, you can even do CI. I'll list a few.
The one I'm doing atm is a Life BM Elementalist Witch, which I believe to be the most well rounded, and has the best clear speed once you have good gear (Proliferation OP).

I went with Conflux first, then Proliferation, then Golems, then Reflect Resistance. In terms of leveling, I was Firestorm until level 50 or so, but it really depends on what gear you have - you can switch to EA as soon as you have Blood Magic and enough Regen+Life. Leveling order depends on what skill you're leveling with...

Elementalist brings reliability to the table with Elemental Conflux, Proliferation, Extra Golem Effect (Crit + Accuracy), and relies on gear less for Resists and damage.
Berserker has excellent sustain with the War Cry ability and overall best damage if he Ignites (less reliable), and is squishier.
Champion is the best in terms or Life/Damage balance, beating Witch in both eventually, and coming close to Berserker since he can give up Life for Damage without much loss, but draws Agro and has less reliable Ignite.
Raider is Clear Speed, with very high Effective Life (Acro + Life).
Pathfinder is Clear Speed, can use flasks for Block, AoE + 3 Projectiles, Move Speed, Armor, Max Resist, and all the other flask OPness that GGG has chosen to allow... for now.

CI builds get to use Soul Strike and win the game if they have enough ES.

I've listed a few other niche EA builds I've seen work or have tried. Most rely on Ignite nowadays to work their single-target magic, but not all.

BM Elementalist (My Current)

BM Chieftain

BM Duelist

BM Berserker

BM Pathfinder

BM Acrobatics Raider

Crit Acrobatics Mana Assassin

CI Elementalist/Occultist

MoM Pathfinder

MoM Berserker

Grand Spectrum BM Acrobatics Vaal Pact Berserker

https://www.pathofexile.com/passive-skill-tree/AAAABAQDAJstXzmK8MSCsKu-p2VNvTZHfoTZQzGNfre2dcvTby5T2sF07QUt6mIZtC2DFSAs6Yaz3Q1brxR15eacxLcivJ9aGuxcSsiKr5ARe8MB3HfjqW7AGm87NuhYr4ZgOlIOPN-_20_gwyW8829J1Qm7FHH46_PdaGXqGPcycqmJ2GhY15YfAgQHTLMnL5BVGjhmVJRvcg9Z86yqVcaX9O_rl5U9X1JT6-4c3I8apwiPRlH7zRYfGLiTKPqYrYLH-ehodNpiGFZBh7c-_grYvYPMPC2DX8MzGJGD24umG0behzbF8CZWSNgk0B81krXyeA1uqoSUiXF71A==

or

https://www.pathofexile.com/passive-skill-tree/AAAABAQDAHvUiXGElOxc3ofwJoumqW7AGm87eA218jWS2L1uqkrIiq-QEXvDPycqTX1brEdRTMpKb56DCWBDbj2-gF-wm7WExcM6G0Yi6ujWBbUajVsmf_uNv9QjMHHdqEt4OkLC7LTFDPKly2KsbIxRRzfU7T8-z73mTZIj9o19MHz-Crc-QYf56Gh02mIYVoPMPC2DX1ivNuiGYPNvSdUJuzpSDjzfv9tP4MMlvLyfGJGCxyj6uJMc3I8amK3r7prgRZ1TUqcI2CRWSI9GUfvNFh8Y8YqGsyzpAO5sCNvnaGXz3fjrFHF34wHchNlHfr02ZU2-p7CrxIKK8F85my0=