Here's how to play Nunu based on AP and Health.
There are few recommendations to do...
1 - Ice Blast: Try to always use it to harass enemy particularly at the beginning of the game (You should not be in mid).
2- Blood Boil: Try to keep it active as much as possible about you and your mate in the same lane.
3- Absolute Zero: As soon as you reach the level 6, try to place the first kill (or better a DK), using a good combo using Ice Blast and Absolute Zero. Pay attention to do it with stunner away from you
Always use the Absolute Zero when you're sure to eliminate all enemies in range, or heavily supported by the rest of the team.