First of all... Zed is just not recommended for this very reason. Despite what the boards say ("game's over in the first 10 mins"), It DOES get to Late Game quite often. When you get there, you want someone like Lux, Viktor, or Vladimir effortlessly nuking entire teams from safety... not a high risk 1 vs 1 assassin like Zed who has to take high risks/make tons of calculations every time he wants to go for a multi kill.
Having said that...
Assuming it gets to the point where everyone is near full build.
You Split Push and you Split Push hard but DON'T DIE (you watch the mini map, and save the shadows to run, you can already clear waves without your shadows, just Qs and Es will do for hard pushing). Get all the Towers you can get and keep making them come to you (you then run away or pick off whoever you can). DON'T DIE when doing this. Eventually, you'll get 1-2 EXP lead over their team (from all the solo minion EXP) if all goes according to plan and tons of enemy towers will be gone.
In fact, in pretty much all games... Zed should have started Hard Splitting long before late game (at some point in Late Mid game... even in the most favourable comps, splitting is just better). The thing with splitting is... NEVER die. Its just constant hide and seek for you.
If all/most of the Tier 1 and 2 enemy towers are gone and you have 1-2 EXP lead... group up with your team and try to get those Multi Kills using your EXP/gold lead; buy Mercurial if necessary to counter some CC. After 2 team fights... you'll probably have either have won or lost that game but its probably your only shot.
...
However, at some higher elo they will send a TP duelist top (who will save TP to force 5 v 4 when you split) to "match" your Split movements while drafting a CC heavy comp to stop you (and arm multiple Zhonyas/Stopwatches). This is your absolute worst nightmare scenario.
What you then do is you Hard Split, but you make sure there are periods of time where you are Off the enemy radar completely. If the TP Top is away at the wrong place... you quickly join your team to kill their key targets before his TP sets off. You then have to keep timing your Splits with your estimation of his TP cool downs.
Yeah... just don't play this champion... its 300% easier to team fight with someone like Viktor at all eloes.
