I'm gonna be upfront and say this is my first guide I've made so cut me some slack and give me some feedback on it! The main focus of this build is using the mushrooms for dealing massive damage. I'll go through where to place them and how to avoid Oracle's Elixir.

Pros

Deals loads of damage with shrooms and toxic dart
Easily counter jungle
Secures runaways easily

Cons

Very squishy
Weak early game
Hard to master shroom placing

With runes i like to focus on magic penetration and ability power.

I use 22/5/3 for damage and that extra bit of health and speed.

Go for Barrier and Exhaust use barrier when in a tough situation and exhaust when chasing. (Should be obvious)

Always go for Liandry's Anguish first then go for Sorcerer's Shoes , Go for rabidon's deathcap and Void Staff. Finish off with Zhonya's Hourglass and a Rylai's Crystal Sceptor .

Basicly judge whether to get Blinding Dart first or Toxic Shot based on who're your against. For champions like Nasus Darius or Tryndamere go for the Blinding Dart, If its someone like Ryze Vladimir or Heimerdinger.

Max out Toxic Shot first and your shrooms when you can. Max out Move Quick last.

Above there are red marks, these are some places you could put musrooms, try to place in their jungle if your team has pushed, or keep it in yours if you've been pushed.

Also place at dragon and baron, but the support should put wards there anyway.