A heavy crowd control team helps Nunu & Willump ult for the full duration unleashing tons of damage and a possible instant penta kill if their team is stuck in it for long enough. Such champions like Amumu, Malphite, Blitzcrank and Thresh help massively decreasing the chances of the enemy canceling it.

Possibly have someone top like Kennen or Vladamir who can easily switch with you before game if you are being countered and having a hard time. Make sure to take mid if there is a squishy opponent like Teemo or Kennen.

Make sure to start with Amplifying Tome and a Mana Potion