If you invade Nunu, do not fight him when he is near a jungle camp as he can use to gain a health advantage. Ideally, fight him when he is in the river or not near a camp.
Nunu can easily solo the Dragon at level 4 and after. Be prepared for this by having the Dragon warded at all times and by always taking the bottom side Scuttle Crab.
At level 6, Nunu can channel his . When he does this, make sure you cancel or interrupt it. If you can interrupt his channel early, his will not deal a lot of damage and it could save you or your allies life. Cancel his with CC.
When it comes to securing an objective like the Dragon or Baron, never go for the 50/50. Because of Nunu’s , he will win every battle. Try to take the objective when he is dead or on the other side of the map.
Nunu is rather immobile when it comes to team fighting (once the fight has started). Lock him down with crowd control and kite him during the team fight so he is unable to be the frontline and get on to your carries.
Do not clump together when it comes to team fighting. If you stand close to one another, he will use his Snowball () to engage from afar and run into the enemy. If you are all close to one another- he might be able to get a 5 person knock up. Instead, group but not to closely.
Nunu can take an early Dragon after level 4. Keep the Dragon warded so your team can collapse on him when he tries to take it. If the first Dragon is an Infernal Dragon, put high priority on it and try to take it instead.
When Nunu has Predator available, he can easily engage from afar with his . As a laner, do not over extend as he can easily gank your lane. In the mid and late game, ward the flanks so he cannot engage with his either.
If Nunu and his allies are ahead, they can easily take the Baron on the dot of spawn. Just before the 20 minute mark, ward the Baron so you can see if they are trying to take it. If they do attempt it, try to collapse on them or push them away from the objective.