TLDR: Resolute Technique + mana efficiency solves accuracy and mana issues from 3.15. Chieftain now possible in Trade with Iron Will.

To those who loved this build and knew friendly company therewith: this Reunion is for you.

Spellslinger Flame Wall Elementalist was an extremely popular build in 3.13/3.14. It was one of the strongest league starters featuring extremely smooth gameplay, and some redditors might have come across me recommending that build to others back in its heyday.

Unfortunately, 3.15 brought a quadruple whammy of nerfs to golem buff effect, mana reservations, Flame Wall damage, and trigger costs, and the build was no longer considered a viable league starter relative to the other choices in the meta. 3.16 made further changes to elemental DoT that seemed to drive the nails deeper. Elemental Overload and Elemental Equilibrium went away, and while most pure elemental DoT skills were buffed to compensate, this did not reverse the nerfs of 3.15.

Accuracy

That said, the biggest problem facing the build was the loss of free accuracy from Ice Golems. Accuracy is required for the projectiles that pass through the Flame Wall to hit enemies and to apply Flame Wall's debuff. If there is a significant (>10%) chance to miss, then the build just feels bad because your damage just fails that often.

However, now that Elemental Overload only benefits the skill that crits, we no longer have access to Elemental Overload to boost Flame Wall's damage. Since we no longer need to proc Elemental Overload, we can now make use of Resolute Technique to guarantee hits on enemies. Conveniently, Resolute Technique is only a few points past Divine Judgment, and near enough to Tireless, Arsonist, and Barbarism that it is no hardship picking it up.

Rebirth

With the biggest mechanical problem facing the build solved, and its mana issues ameliorated with masteries and mana efficiency changes, this opens the way for Flame Wall Spellslinger's revival. Granted, the numbers are not as good as they once were (about 200k Sirus DPS at lv90 with just the tree and Ashcaller as a placeholder weapon https://pastebin.com/m4WkRDp3).

However, there are tools to counteract this: Witch has a very nice 27% DoT multi wheel, 20% FDoT is now also available on the tree, and weapons and jewels can roll DoT affixes which should bring damage up to a reasonable level. The spell damage boosts from Righteous Fire and Vaal Righteous Fire can also be sustained with the aid of Flame Golem, Vitality, Rise of the Phoenix and some added max fire resists on the tree, and even further with the Recovery on Ignite fire mastery and possibly also the 2% life regen while moving mastery. Good weapons and decent investment can bring the damage up to about 700k Sirus DPS, which is quite enough for all non-major bosses.

There is also the option of using Shaper of Storms instead of Primal Bastion for another ~20% boost to damage, since Divine Judgment gives access to 60% increased ailment effect, and reflect immunity is not a huge concern for a DoT build dealing relatively poor hit damage.

Defensively, Stone and Chaos golems still provide a decent amount of (static) defence. The build also has the option to spec into some block chance, and it's near enough to Glancing Blows to pick it up, but its main defence is still going to be murdering everything three screens away.

Coming out of the acts (https://pastebin.com/ZPaJFxFc), the build feels as good as it once did, which is pretty promising. Kinetic Blast is now the only wand attack worth considering for the build given its extreme coverage, as the Elemental Hit + Elemental Equilibrium tech is no longer accessible nor profitable. Power Siphon is somewhat acceptable as a substitute before you can acquire GMP in A4.

Levelling

In terms of levelling, the main idea is to run Flame Wall + Holy Flame Totem till A3. Afterwards, you can transition incrementally to Flame Wall Spellslinger as you get the necessary pieces.

• A1: Pick up Flame Wall, Combustion, Scorching Ray + Spell Totem, Clarity, Vitality, and Flame Dash
• A2: Pick up Wave of Conviction, Herald of Ash, and Controlled Destruction + Elemental Focus.
• A3: Pick up Spellslinger, Flammability, and Kinetic Blast. Pick up Pierce (and optionally Swift Affliction, Burning Damage and Inspiration, depending on socket colours) from Siosa.
• A4: Pick up all golems except Ice Golem, and GMP from Dialla. Pick up Hextouch from Petarus/Vanya as well.
• A5: If you opt to run Hextouch with KB-GMP, you need Poacher's Aim from Bannon to fix your Pierce. You can go with Assassin's Haste for the mana and movement if you run a WoC - Hextouch - Flammability instead, in which case your KB link should be KB - GMP - Pierce - Combustion. IMO Wave of Conviction is not great until you can get at least Mastermind of Discord or a trigger wand (which also necessitates a Flammability curse on hit ring); the exposure link should be Spell Totem + Scorching Ray till then.
• A6: Pick up Infernal Cry from Lilly, and use it on bosses.

Lab order is Liege of the Primordial, Elemancer, Mastermind of Discord, and then Shaper of Storms/Primal Bastion. Liege of the Primordial and Elemancer (alongside Golem Commander) are necessary to run Chaos, Lightning, Stone, and Flame golems for pretty good QoL in general. Mastermind of Discord permanently renders mana issues moot, and Shaper of Storms/Primal Bastion is the cherry on top either offensively or defensively, without being too vital to the build.

The tree order is to rush up towards Acrimony, then left towards Sovereignty and Divine Judgment and down towards Resolute Technique. The order of notables/keystones should be Divine Judgment, Resolute Technique, then Sovereignty. Afterwards, path towards Heart/Breath of Flames, then take all life nodes to pad defences as necessary.

Chieftain Flameslinger

The addition of the Iron Will keystone to the tree, as well as the pathing into Marauder regions to use Resolute Technique, opens up a new possibility for Flameslinger as well: Chieftain (https://pastebin.com/zLheJwTf). Without golem buff effect being as crucial to the build, other ascendancies are now comparable options (including Inquisitor - but without a good fourth ascendancy - and Trickster - but requiring some shenanigans like Skin of the Lords or possibly a krangled body armour to obtain Resolute Technique), but Chieftain is definitely the best of the lot.

Chieftain is remarkably well-suited for spell fire DoT, with Ramako, Hinekora, and Arohongui all boosting fire damage done, and Tasalio making Righteous Fire really cheap to run. The idea is to go strength-stacking and rely on Iron Will and Iron Fortress to make up for Chieftain's lack of spell damage, and for Tasalio to support both Righteous Fire as well as make it easy to run Formless Inferno for a significant armour boost.

The Chieftain has two major gotchas for levelling as a Flame Wall Spellslinger, though. First, he doesn't get many of the necessary gems, requiring access through Siosa in act 3. It is also further recommended to mule a Witch to the Submerged Passage for the Flame Wall gem early, otherwise you can only access a lv1 version of it from Siosa. Second, a lot of travelling is required early on with no good damage nodes, making getting through A1 to A3 and to Siosa somewhat of a pain.

The only major change from Elementalist to Chieftain will be the exposure link, which must be Spell Totem - Scorching Ray. Not only is this 66% more exposure than Wave of Conviction, the totem is necessary to proc Chieftain's Arohongui, Moon's Presence ascendancy notable, which is very strong offensively (% increased fire damage taken by enemies) and defensively (enemies near totems deal 8% less damage). This forces Combustion onto the KB 4 link with GMP and Pierce, which means Flammability is best handcast and a curse on hit ring is a bit more of a priority.

Lab order for the Chieftain will be Ramako, Arohongui, Tasalio, then Hinekora. Ramako comes first for the free 25% fire DoT and ignite chance for Combustion proccing. Arohongui capitalises on the Spell totem, whereas the bonuses of Tasalio are most relevant at the beginning of maps to relieve suffix pressure (especially in a strength-stacking build), buff physical defences, and pave the way for running RF. Hinekora is an afterthought as its only real benefits are the 10% increased strength and Covered in Ash; if you really must, run Infernal Cry until you can get that last ascendancy point.

Tree order for the Chieftain will be hitting Warrior's Blood, then pathing through Barbarism to reach Resolute Technique. From there, path up one and straight right to hit Iron Will and Arsonist, then pick up from Resolute Technique to head up to Divine Judgment, Sovereignty, and Heart/Breath of Flames. Once done, come back to the Chieftain start to pick up Soul of Steel, Bloodless, Call to Arms, Utmost Might, and Inexorable, then cap off with Prismatic Skin and the various life and FDoT nodes in easy reach.

Conclusion

In general, the Elementalist is easier and cheaper to run as a league starter, enjoying higher damage and comparable defences without investment thanks to the Chaos Golem providing significant PDR. However, the Chieftain holds the promise of an extra damage scalar in the form of Strength, which also makes him tankier (especially when combined with Iron Fortress' block per strength). Both builds are now almost exclusively mappers: it was formerly possible to boss with it, and it still is possible to raise their DPS up to 1m+, but that would now take a greater amount of investment which may not be to everyone's taste or priorities.

I would recommend again the Elementalist as a generically good league starter for both trade and SSF. My experiments with it (and the key realisation that RT is now viable with EO's exit) came too late for me to leaguestart it this time, but I'll still be rolling with it as a Legion farmer for my Reave Berserker and Boneshatter Juggernaut, as it's the fastest and cheapest clearer that I have access to.

I'm not so sure about the Chieftain's place though: it just might require too many uniques to be truly SSF-viable (although it still works, Elementalist is better if you cannot get strength stacking equipment), and in trade, the uniques that it would want are the same as other strength-stackers which are increasing in popularity. Since it feels like most other strength-stacking builds would scale a little better than Chieftain Flameslinger, I don't think Chieftain Flameslinger is sufficiently competitive as a meta league starter in trade, but I'll be happy to take trade players' opinion on this as I'm primarily an SSF player.

All PoBs provided are sketches and by no means end-game ready, but feel free to add on your ideas to the general framework laid out. I'm pretty confident I haven't missed any major tech that would drastically affect the build, but in case I have do let me know as well.

EDIT:

Postscript - Comparisons with CF Gladiator

I've since gotten comments that for a similar playstyle, Gladiator CF is now the way to go. I've had a brief look at the general archetype and I sort of agree insofar as bow + quiver is the best way to load up on a whole load of DoT-related mods that no other weapon combination can compete with. And of course the numbers for CF are going to be better, as you free up Spellslinger's reservation for another damage aura as well as Spellslinger's socket itself for another damage support.

To at least be able to report on a fair comparison I'm levelling a Gladiator CF build myself - I've taken some general inspiration from Grimro including his reservation setup, but I'm choosing to go with wand + shield instead, and to swap in Power Siphon + Barrage instead of KB + GMP for single-target. I'm also going Lancing Steel as a temporary carrier while I wait for lv38; so far it seems like a good carrier with Pierce.

That being said, other than bow + quiver being the best weapon combination, some of Grimro's tech is worth looking at to bring FDoT including Flame Wall Spellslinger to the next level. For investment, we could get Malevolence in, with a helm enchant, or even Enlighten. Cluster jewels with DoT multi would also help a lot. + gem levels are of course an excellent way to scale, with corruptions, wands, and amulets. The main thing is trying to find a way to reduce the mana reservation cost to allow more auras to be run, which is mainly what is carrying Grimro's CF damage (as CF doesn't innately benefit from spell damage unlike Flame Wall).

In any case, this post was never about Flameslinger becoming really top tier meta again (although the title is perhaps a little misleading in hindsight), but the substitution of Resolute Technique for Elemental Overload solving one dimension of the problems that Flameslinger faced after 3.15. I am heartened by the comments from people who have played this build previously, have enjoyed the mechanical feel of the build, and are glad to know that it is at least still numerically viable. It is these players whom I made the post for. I'll update this post in a while after I get that Gladiator levelled so I can report on the feel of the build and whether it is the next Flameslinger as far as mechanics goes.