with my main acct, I have around 650K zed exp...he's really an awesome assassin.
But playing an assassin is a whole different playstyle, you want to be the last to engage in a teamfight, Not the first, not the 2nd. Once the teamfight is initiated and everyone blows their ults or become scattered, you go STRAIGHT FOR the enemy adc, ult them, kill them. Then depending on your situation, move on the target next to you, or jump back to your shadow with R. There are also really creative combo techniques with zed. My personal favorite is the "Line" where you ult the enemy, throw your shadow behind them. Then do your full dmg rotation. so if zed is x and enemy is O it looks like this. x xO x and you then have more combat options, If the enemy JG appears you an jump back on your ult shadow. if they try to run you can jump forward to your W shadow. zed is really the most versatile assassin in LoL