Except all you have to do as Nunu is:
Steal the Main Camp and land a Q on Yi while he's trying to farm it so the Monsters might actually help killing him.
The idea isn't to duel Yi, but to get him low while taking his Camp then walking out (hopefully towards your Team collapsing so if he chases he dies to the Team).
So wait for Yi to be low in Early Game to a Camp, Snowball him then Consume the Buff after it lands another big hit, then walk out. Yi loses Gold and Experience (and Stacks on Sated), falls behind and becomes useless, never to be seen again because he's power farming to catch up.
Now, you don't close out the game... he WILL catch up and you will pay for it. Which is the real issue.