In depth tutorial on how to play the new Cassio
Step 1: try to poison someone
Step 2: Mash your E like your life depended on it
Step 3: Build Avarice Blade and Frost Queens Claim. Trust me, you won't notice a difference between having those items or a Deathcap since your damage is meaningless before 400 stacks, you seriously won't. So you might aswell get the gold instead.
Legit, I got from Bronze I to Plat V with this guide and I hope it helps you too.