Nunu has many options and because of this build nunu has a better way of sustaining and pulling his own weight.
When starting off a match i prefer to begin with Buff by adc's side of the map since it can be killed faster and give Nunu the advantage early on. Since Nunu is weak early on is best to avoid invading and ward your own jg and if it were being taken ask for assistance since most jungle prove to hurt nunu badly early on. Anyway, as soon as i am level 3 i gank whatever lane is of need for most help. I start with W from a distance and if warded your speed would still be significant enough catch up to them. Once hit immediating follow up with your E and keep them within the circle so they are rooted. By then Electrocuted would of activated and they would be low yet Nunu's Q wouldnt be strong enough to finish them which then makes you reliant on the laner to assist you. If the gank goes wrong, your can usually at least rely on your e to root them before running and use your Q on a minion or them to heal up. Level 6 onwards we Begin with the W and and use e immediately as soon as the third e is thrown and the enemy is in circle, Press R this will slow them further and they will become rooted from nunus E. And so the R will finish them. Because of the luden's echo on the Jg item nunu begins with a lot of early gank damage and so I continously gank lanes farming the red, blue and dragons.

Late game Nunu Becomes weaker and doesnt do as much so i like to stick with my team so that all of nunu's CC comes to use.

So unless the enemy jungler successfully invades your jungle early on, repetitively, Nunu will carry the game early on.