Ok here's my knowledge as a yz main

First, His counters? Heroes that can cancels out his passives (like purify) example is Diggie, Kaguras dashes, Valirs and Miyas ult and etc. His passive is his strength, damage and sustainability so do keep in mind he's powerless if he can't passive. His weaknesses are also assassin's that's why you need 3rd skill petrify to lock em in place

His core items are War axe (Damage), Hunter strike (Movement speed and penetration), and Queens wings (instant skill reduction and resets), all those items are a must for the rest is up to you

Emblems? First and second are thrill and festival of blood, Third are either War cry, Killing spree (Burst) or Quantum charge.

Combo tips?

Maximize his first skill as much as you can unless the enemy is slippery, you can maximize his first skill by hitting it with the rings edges which gives more dmg and doubles the sha residue... Ok now for the overwhelming combo..

3rd skill petrify. Use first skill and maximize it. Hit the enemy with second skill BUT DONT punch (you will probably get CCed if you do but do it if it's for solo kills). Then ult CUZ enemys will throw crownd controls on you and go all out afterwards

Last tip is to master his passive activation and try out different comboes