BURST

Your passive for your E as thresh Flay, scales ridiculously and provides so much burst you could pop a country's economic bubble.

"Passive: Thresh deals bonus magic damage on each auto attack. The damage is equal to the number of souls he has collected plus 80/110/140/170/200% of attack damage, based on length of time since last attack."

200%

With the items above you would have 80% crit. You can sell boots late for phantom or something to get the full l00% but I don't normally do that.

Stack this with Statikk Shiv and Rapid Firecannon and you are now able to 1-shot a squishy champ. ADCs in 2017. As you farm you gain AP and Armour, plus extra damage on your E and its passive. You, as an ADC, now have engage, disengage, crowd control, tankiness and burst. You also scale with souls you farm, and as an ADC you should have a lot of farm. You're basically a ranged nasus. Or an AD veigar. Almost as if you were Vayne and Sivir's love child.

Late game you kind of fall off a bit but thats where the Frozen Mallet comes in. Frozen gives you 700 health, coupled with all the souls you've collected you should have a decent amount of armour. Get Blade of the Ruined King for attack speed, percent health damage and lifesteal. Maybe even disengage by using it to steal movement speed and your mallet should slow them down sufficiently.