When playing Nunu, you should focus on doing aggressive ganks and getting the epic monsters (drakes, haralds, baron).

I usually gank immediately at level 2. Most people aren’t expecting a gank that early in low elos, and your level 2 snowball/W damage is really high.

A strong strategy is telling the midlaner to get pushed in so I can gank at level 2.
Many players in low elo just push the wave in and dont think about the consequences.

It’s important to utilize your w to clear your camps, it really helps you clear speed.
For example raptors get cleared almost twice as fast if you w into them. Nunu is very hard to late game carry with, so it’s really important that you try to gank your lanes as often as possible to get them snowballing (xD) into the late game so they can dominate. Also, if you gank a lane, and notice that the enemy flashed, immediately come around to gank them again when the lane is pushed in.