Do you Want that mastery 7 so hard but you keep getting S- or A+?
Thresh jungle does not work for you?
Tired of being insulted becouse you dont gave the kill to your ADC?

HI! im TwentyZero and if you are reading this, im the answer to your questions.
I PRESENT YOU THE MIGHTY ATTACK SPEED AND 100%CRITICAL CHANCE THRESH , THE LASHING BOSS !

Let me explain a little about this, You are not a support or a tank, you are like a crazy ADC, so your paths should be like an adc.
I choose Lethal Tempo becouse this rune helps in your early trades becouse NORMALLY the people will subestimate your damage output ... and when you start giving those hits with the pasive they will panic.
Also try to grab'em under tower to benefit yourself with your habilities and the outplay power you have.

Triumph Is a common rune for an adc becouse ,it gives you life and gold!
Legend: Alacrity gives you MORE attack speed, for those tanky enemies that have long habilities coldown and squishys that like to run away from you.
Coup de Grace will help you in early trades when you aint have any items

In the second Tree , its a little bit about your playstyle , i go Sorcery becouse of the Manaflow Band that helps you with the mana issues.
Gathering Storm its .... a choice,i use it to keep scaling with damage to late game.

You can go Resolve in the second tree to give yourself a little more tankiness or to help you destroy those Towers fast.

I would like to remark that i never used Dark Harvest but I Think it could be a great pick becouse dark harvest gives you a boost on your first attack. Yeah, you read my mind. The passive of Flay works Exactly the same! the more time you dont hit someone, More damage you will deliver, imagine this : you are running to your enemy, you got your passive of flay, dark harvest and Stattik full charged..... nasty dont you think?. I will test this in my next game!.
Also i would like to say that trhesh farms pretty well with his pasive.
ACTUALIZATION:
I tried Dark Harvest and i realize that this item is more of stacking to late game , and This build im showing is for early-mid spike ,so it doesnt work!:(

Dont use Press the Attack becouse you are building full Crit and attack speed. the fast you apply those crits , the better and in early trades it wont help you becouse you will not overpass the damage of many toplaners.

Path 8.11

In this path i suggest you keep with lethal tempo. You can use Predator for the bonus damage and movility , many are using this path, OR Hail of Bladed becouse you dont want long fights but also you need to apply those CRITS fast if you dont one shot. However ,i think electrocute/lethal tempo its still okey.

Pros

-Great attack speed
-Easy Farming
-Hook
-Outplay capabilities
-Shield for you! also lantern can save your ally if gank fails.
-Ranged Attacks
-You scale armour and damage with the souls
-Strong against tower dives, your habilities will make them think it twice
-Mid and Late game squishy deleter
-Magic and physical Damage
-ULTRA FUN

Cons

-You are no more a tank, so Watch out!
-Weak againsts bruisers
-Weak against ganks
-Weak against ranged champions
-You push Too Much the lanes againts bruisers or tanks.