Update from this weekend's farming, and my experience overall:
Build is strong, but not cheap. Even on a budget, expect to spend at least 2-3 ex to map comfortably.
The Watcher's Eye is absolutely NOT mandatory. I took the Fenumus Spinnerets + Spider combo all the way to T16+ without issues.
However, having the Watcher's Eye + Fenumus turns you invincible when paired with a Fortify Mjolner. Have not yet tested just having the Watcher Eye, as I like having 50ES/Hit (might be overkill but who knows?)
How tanky does this get? To give you an example, I've fought the ERADICATOR 4-5 times now. In tier 16 maps, with nasty mods.
Every single time, I am able to FACE TANK his entire "Storm Burst" spell (The one he floats up in the air and shoots down dozens of projectiles). Can facetank all bosses, including Sirus. Facetank Shaper/Elder etc. T16 metamorphs with MULTIPLE unique components - face tanked. I don't even move for any of their effects. I ignore all the adds, cause I can just sit on the metamorph until it dies. That Caustic Ground that gets life characters killed? Well, CI makes you immune!
Basically, anything that isn't a "huge one shot" is not going to kill you - as long as you have something to hit, you are essentially at PreNerf Vaal-Pact levels of instant health restoration.
My gear isn't perfect, but it's not cheap. Only at around 8200 ES which I find is sufficient for all content. I would prefer 9-10K to be truly invincible to even some oneshots, but will need to save up more $ first, and also need around lvl97 to fill out skill points i want. Overall, DPS is more than sufficient, as is EHP sustain - the key is to just get more EHP so you can survive oneshots to leech back up.
(PS for those wondering, I've played multiple CoC Cospri builds, and I think this build is far superior for single target/sustain through many hits/second and ES on hit. However, Cospri CoC probably has better clearspeed as frostbolts travel farther and Novas can appear on frostbolts, plus Nova has bigger AoE (hits everything around you). CoC Cospri also "sounds" and looks cooler with all the ice/freezing/shattering, plus allows you to use Herald of Ice which is awesome. But for this league, where single-target bossing is emphasized (metamorphs) and you want to be tankier, this build seems superior. Clearspeed is still VERY good. Much better than Mjolner Arc.)
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Working on minmaxing/crafting better gear. Good build from OP but I believe there's some room for improvement.
Some issues/fixes I've noted:
1) A lot of players still using Elemental Focus in both BL setups. I highly recommend dropping at least 1 so you can shock - it's hard to calculate exact DPS with shock via PoB, but it's now easier to shock - so even on bosses I believe the DPS is higher with something like Lightning Penetration (or if you can afford it, Awakened Added Lightning). I think using an Innervate gem in your Mjolner is ideal as the damage boost (buff) will also apply to your main 6L CoC BL for maximum damage. It will always be up, even on metamorphs since they spawn trash - just swap it out on stuff like Sirus since he has no adds.
2) *** HELM ENCHANT
40% BL damage is recommended but I don't believe this is strong or optimal. 40% increased damage is not that much.
I think the best helm enchant by far is "24% Increased Ball Lightning Area of Effect"
It is important to understand that Increased Area of Effect is actually a very strong stat for BL - even though it doesn't benefit from AoE damage. The key is that increasing AoE will increase the "duration" the ball is within range of your target, so it gets off extra hits. If you look at some Ball Lightning Miner builds, a lot of them take Inc AoE for this reason. The key is to hit certain "thresholds" that the ball can zap a target 1 more time. (If, for example, the ball normally can only hit a target 4 times before it's out of range, and you increase the AoE just enough so it can hit 5 times, that's a 25%
MORE DAMAGE INCREASE - HUGE)
This build can take 40% inc AoE from the tree, which is a big DPS boost. I checked some calculators and adding on an extra 24% from the Helm Enchant brings us up to a higher breakpoint, allowing more hits, and is a big damage increase. Like 10-20% more damage - way more than 40% increased.
According to various calculators online, such as here:
https://www.reddit.com/r/pathofexile/comments/6xz7rs/finished_my_ball_lightning_dps_calculator_that/
Not to mention it will also provide some more QOL for mapping.
3) Timeless Jewel -
I have not tested Militant Faith yet, will do that next. I think with Disciple of Forbidden + extra power charges + Militant Faith you can reach huge DPS. But in general, I feel like DPS is already enough, I prefer to be really tanky.
So far using Lethal Pride seems strong, but instead of using it just for strength, I prefer having it by a keystone to allow for the block bonus.
This build essentially needs to survive 1shots to leech - having 75% attack block at all times for 50% reduction is very strong, since it smooths out damage you take. For 1 skill point (on a keystone) you get 30-35% block which is quite strong.
You can even get more spell block if you want, but I don't find it necessary.
4) SHIELD -
With the above Lethal Pride, coupled with Light of Lunaris, 75% block, equals 75% crit multiplier. The shield gives 25% extra mult on non-crit (almost always up since you will not always crit with Cyclone) the shield itself gives 100% crit multiplier.
In addition, somewhat overlooked, but the shield provides 40-60 cold damage to spells. While this may not sound like much, it's increased significantly from our spell dmg and crits, and is enough to FREEZE AND SHATTER almost everything I kill. Those annoying crab mobs that die and then spawn without their shell? Well, they die instantly with this shield, because we always crit and they will always freeze and shatter.
Lastly, it has good block. With just this shield, tempest shield, and Lethal Pride i have 75% block chance.
I tested compared to a well rolled shield with 70%+ spell damage and +1 lightning gems - Light of Lunaris is still #1 for offense. DOWNSIDE is that Lunaris only has about 170 ES so its much lower for EHP compared to a good shield. If you can afford like a 10ex+ shield with 300 ES and good offensive stats it might be better, but budget shields will likely be weaker - and also won't have good block.
** BONUS - Using a unique shield makes it easier to shield swap, since your resistances won't be on your shield. If you want to improve clearspeed, you can swap Lunaris shield to Zeel's Amplifier - offers 50% increased AoE for clearing trash, good for lower tier maps if you don't need max DPS etc. Remember that Inc AoE also increased Cyclone AoE (not just BL) so you can hit things without having to move around as much.
5) HELM -
I picked up a +1 power charge Crown of Inward Eye for 1 ex - they seem pretty common (as of typing this, I see 2 on trade for 1ex each). This is obviously quite cheap for the power they offer - a well rolled rare helm with -9% lightning and good ES will cost at least 3-5 ex. The ES is pretty low, and unlikely you can find one with a Ball Lightning enchant - but +1 power charge on this helm with built in Tranquility is pretty good. This way I can allocate Divine Judgement on my amulet for some extra DPS.
Eventually I think best helm is still 24% BL Increased AoE with + lightning damage to spells, and -9% lightning damage to nearby - but this is going to be 10-15ex+ with good ES rolls.
6) ONESHOTS
The biggest upgrade for me wasn't actually getting the Watcher's Eye, but getting a Fortify on Hit Mjolner. NOT CHEAP AT ALL. But 20% less damage taken at all times, alongside my block, really helps "smooth out the damage" to avoid 1-shots.
Yes, you can run Leap Slam/Shield Charge/Fortify before you get it, but even being a veteran player, this is not as consistent. Having to stop cycloning to leap slam/charge every X seconds is annoying as not only can you forget, but it leaves moments of vulnerability where you aren't hitting to leech, and you can be frozen/stunned between cycloning.
Alternatively if you are suffering from being squishy you can consider swapping your Curse on Hit to ENFEEBLE. You can, for example, pick up a Shaper's Touch with Enfeeble on Hit for 70c - 1ex, which isn't that expensive, and enfeeble is very strong for reducing damage. Or even pick up +1 curse on tree.
7) LEECH
While this is an ES-On-Hit build, I noticed a considerable deficiency in regular ES Leech. PoB shows 148 ES Leech/Hit (each instance) with leech cap of 1630/second.
Allocating just 3 points into Light Eater increased this to 621 ES Leech/Hit with leech cap of 1956. That's a big increase from prior number, making it so i hit my leech cap very quick (like 3 hits) instead of 11 hits.
Since i'm using a Watcher's Eye with 30% ES Recovery (you can also get this mod on belts, etc.) it bumps my leech cap up to 2467/Second.
Overall I am not sure how important hitting leech cap is quicker, and we already have plenty of ES/Hit - but I like the idea of 2500 ES Leech/Second + 575 ES Regen (Consecration + Zealots Oath) alongside ES/Hit. Makes me a beast of a tank.
8) AURAS
If you are too poor to afford Enlighten 4, build functions very well with Enlighten 3 (less than 1 ex) - put 4 points into Sovereignty cluster. This allows you to run Zealotry, Discipline, Aspect of Spider, and high level Precision (I used lvl 12-13).
Once you get Enlighten 4, you can save 3 skill points. Downside is really low mana pool, makes flame dash/vortex difficult to spam.
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Overall happy with build and will be sinking in more currency to minmax in new week. Biggest upgrades I want are:
1) Chestpiece with 650-700 ES and special mod. Unsure what I want yet, I've been aiming for +1.5% attack crit (to maximize CoC) but there are some good mods now with new influence. Frenzy Charge on Hit may be strong but then we'd have to drop attack speed to stay under 6.0, or maybe +1 curse.
2) Helm with high ES, lightning to spells, -9% nearby lightning, and resists - Still unsure if this is BIS, since +1 power charge on Inward Eye + Disciple of Forbidden is strong. But I believe 24% LB Increased AoE on the helm will push it to the top, if you can afford it.
3) Maybe craft a good rare boot, although losing 18-20 ES/Hit from Spinnerets might not be worth it. Also don't forget the Spinnerets allows 4 spider webs instead of 3 spider webs, which is slightly more DPS. Tailwind boots would be sick, but then we'd have too much attack speed.
