If you have a Shaco on your team, expect him to do nothing but split push in the late game

Yes, he's an assassin. Yes, he can burst someone super-fast in teamfights. But is he a teamfighting champion? Hell no. His sole purpose past 30 min is to split push and pick people off. What does this mean for you as his teammates? It means ward the fuck up and don't get picked off yourself, because fights will be a 3v5 instead of a 4v5. It means play defensively and do go running in to engage in stupid fights when you're not winning by at least 20 kills. It means don't expect Shaco to even remotely care about jumping into teamfights, because odds are he won't do jack shit in them. If the other team has a support with even half a brain, Shaco will be spotted by a sweeping trinket or pink ward during a teamfight before getting to do anything. Even if they don't throw down one of those, a Janna can simply drop a 400 HP shield on the ADC and make Shaco useless. Or she could even ult and get him the hell out of there. A well played Thresh can simply Flay Shaco away mid-auto attack. If it's possible to Flay a Rengar out of his leap (who, mind you, can kill a target MUCH faster than a Shaco) then it's definitely possible to Flay away a Shaco. Once he's failed to kill his target, he gets bursted down in under a second, so the teamfight is pretty much a 4v5 anyway. So please, next time you play with a Shaco, learn to adapt your goddamn playstyle a bit. Not even game needs to be decided by a single teamfight in mid lane, or around Baron. Respect your split pushers; if you know how to play with them, they will literally win you more games than every teamfight-based comp in the your entire LoL career.