Nunu is a support jungler. His job during laning phase is to punish slow junglers. You don't pick Nunu vs Lee Sin/Skarner/Shyvana/Mundo. They're too fast. Pick him against Warwick, Gangplank, Etc.
After laning phase your sole purpose is to be the wall between enemies and your carry. Slow anyone coming close, Boil the carry, Ult if the enemy team dives her.
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Realo won gold at the Sex Olympics with a BROKEN FRIGGIN NECK.-Voltch