Hi.
My friends call me Liam.
I started playing league in the middle of season 9. Yeah, I'm still new to this game and learning.
The first champion I got was Ahri (I'm a fan of naruto so obviously the nine tail cute girl was my choice). I bought Nocturne 1 week after. I saw his Ultimate skill. It instantly killed the enemy in the preview so I thought "WOOOW THAT'S AWWWSOME". Well, it didn't killed my enemies instantly :\ I wasn't disappointed though. I liked and like him a lot as my first favorite champion in League. He was my favorite for a long long time until I met this beautiful Angel named, "Kayle". She's my favorite now.
Well... nunu... I first played him on ARAM and the main reason I liked him was his W (I like snowballs). I like jungling with him. However, I don't play him unless I'm sure that I have good allies.

• He got snowballs xD
• His Q deal 960 true damage to monsters at max level. your fights over baron, drake and herald will be easier since you deal massive damage (Q+Smite)
• Lots of healing from Q.
• Has many CC skills. (W - E - slow from R)
• High MS while snowballing (W)
• Good ganks
• Easy champion
• ...

• Weak against CC. (it can cancel his R and W)
• Is a supportive jungler.
• Most junglers can kill you Early game.
  ...

This rune temporarily increases your Armor and MR and deals damage based on your max health which is pretty useful in fights since Nunu & Willump Usually start engaging with their W. This Rune is perfect for him because it works well with his W. The other 2 runes in Resolve group aren't much useful for them so If you don't want this rune, you can change it to Predator for Massive amount of MS. It makes you less tanky but Your ganks will be better. (And more fun)

I sometimes pick instead of this. But is better for ganking and fights. There aren't many chances to get benefits from Demolish but You can benefit this in each of your ganks. And the 3rd rune in here isn't useful for nunu and Willump since their only shield is from Ultimate.

It's the best one here. You can also pick if you feel you're gonna struggle early game. Like if you feel like the enemy jungler is going to attack you in your jungle or you're gonna have early fights. If not, don't pick it.

It works well with your Q and all the healing you get. You can also choose if you want. It can be better if you're focusing on Bonus Health in your build.

Choose the last one dependent on your enemy's attack type.

Remember that your weak alone. Don't force fights unless you're sure you can benefit from it. While clearing your Jungle, remember that You heal 50% more from you Q if your health is below 50%. Don't use your health potions and try to get extra healing from this ability. Use health potions when you decide to go for a gank (lvl-3).

You should focus on ganking, Drakes and heralds. Your Q is stronger than Smite until level 18 so you should use this advantage to help your team. Ganking is important but don't forget to clear your jungle whenever you can.

Support your allies. Nunu is a team player. hf :)