Level 93 Hexblast Pathfinder^3.21

Notes

SHOWCASE
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Mapping - https://clips.twitch.tv/HeadstrongOddInternDogFace-1cNtb5QqOcaaO4U8  
Guardian oneshot - https://clips.twitch.tv/NaiveGlutenFreeGazelleCopyThis-2d4SROAmU4zfCJ-x

OFFENSE
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Simple concept - stack Low Tolerance notable from cluster jewels (300% increased damage with poison against non-poisoned enemy) to apply a very strong initial poison.
This damage is roughly doubled by alchemist's mark which drops caustic ground ~equal to strongest poison. We use Hexblast for very high base damage + more multiplier if removing a curse on a target.

In order to spread the poison we're using Master Toxicist from Pathfinder.

The real trick here is that aside from helping you have comfortable map clear Master Toxicist ALSO lets kill bosses like guardians or conquerors in a split second with very little investment.
We use Vaal Breach to summon a ton of enemies and then draw them to the boss. Once you kill one of these breach mobs with your powerful poison it transfer to ALL nearby enemies (boss and other breach mobs).
As more breach mobs die we pile dozens of copies of our strong poison onto the boss. It's gimmicky but extremely efficient.

DEFENSE
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Suppression cap  
Ele + Chaos res cap  
Grace + Determination + Defiance Banner + Aura effect  
100% uptime flasks using pathfinder + flasks with recovery + some flask nodes on the tree  
stun immunity using stun mod on flask increased by flask effect nodes + ascendency  
further reduced ele damage with saphite + topaz + ruby flasks  
elusive from withering step  

GEAR
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Cane of Unravelling - 2c (extremely efficient poison dot staff. You can replace this with a better staff or wand + shield combo once you have some investment)  
Vixen's Entrapment - 4c (quality of life. You can bulk apply hexes with bane or arcanist brand too but it's much more clunky)  
5 passive Medium Clusters with chaos damage over time - 3c each (this should be your only source of damage. Alt spam/harvest/fossil craft chaos to get low tolerance and whatever else you can manage)  
8 passive Large Cluster with chaos damage or 5 passive voices - 15c. Voices probably better as it lets you get a third medium cluster

Rest is abject garbage I either essence spammed or found on the ground. Amulet was made with harvest chaos reroll. You do NOT need thrillsteel. I was using it for levelling but going fast fun so I kept it.

PROBLEMS
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Without regen or leech the build is vulnerable to DoT effects. You likely want Soul of Ralakesh + Corrupted Blood immunity. Using 4% recovered from flask-use mastery helps a bit and so does 1% life recovered
whenever you curse and enemy (we curse a LOT)

There is also the problem of Master Toxicist's range. "Nearby" is not a fixed value in PoE and unlike Ignite prolif/transfer effects which usually have 20 range or more Master Toxicist only has 15.
You can still clear huge packs in one shot but you'll definitely notice when the poison fails to jump between packs. I don't know why they made the radius so small.

Visual clarity..? Unlike explode/shatter/ignite builds that very clearly destroy mobs poison prolif is just a silent spread. Most maps are very dark and poison is extremely hard to see so it's hard to tell
what's dying and what isn't. I almost wish poison and withering step had their visual effects swapped.

Single target WITHOUT nearby mobs to multiply poison effect is bad. Yes you can oneshot guardians but even that requires a bit of a setup -> wither totem, malevolence blessing, vaal breach AND THEN pull a
bunch of breach mobs to surround the boss and hexblast. It's easy to mess up this combo and only do half damage and then you're in for a bit of a slog.