Zed is a very powerful champion for Middle Lane. Can easy escape and dodge magic with his Living Shadow. Can poke constantly with his Razor Shuriken from a distance and if you come near him he can damage you with Shadow Slash so he is a dangerous AD melee champ. He can poke you from close and far distance and use no mana. Can easily focus their ADC or Mage in Teamfights with his Death Mark.
You need to use your Living Shadow very carefully. It can help you do more damage but its only escape magic. Always care how you use it. You can combine it with Razor Shuriken to do more range damage and more close damage with Shadow Slash.
Because Zed is a melle champ he need lifesteal. Best to buy is Blade of the Ruined King because it gives you lifesteal and AS and you can activate it to slow your enemy and boost your MS. After that you can buy one good penetration item like The Black Cleaver or Youmuu's Ghostblade. After that you can buy Bloodthirster to get damage and later more damage and penetration like Last Whisper and for last hp Warmog's Armor

Zed is very OP champ for a Teamfight. You need to care because he is a good killer for ADC-s and Mages. He can go in with his Living Shadow and target their ADC or Mage with his Death Mark and secure the main kills then return to his previous spot with reactivating his Death Mark and help your team to kill the remaining champs. Zed is often a key champ because he can easy kill their ADC and Mage so you need to use your Death Mark and target their enemy champ very carefully. He can use his Death Mark to break their formation to make them go back while you just reactivate Death Mark and persuade them with the rest of your Teammates while they are Confused and in Panic