I freakin' love Nunu. He's one of my favorites to play. He is generally considered a "situational pick" rather than a top or high tier. I think he's really best used as a jungler now, rather than playing in a lane. He can lane because Consume lets you heal and farm and Blood Boil buffs teammates, but if you look at how he's built, his abilities best fit together in the jungle. Consume lets him farm, but can't be used on champs. Blood Boil and auto-attacking is going to be way more effective in the jungle where you stand around and attack. His iceball is a great snare for popping out of the brushes and his ult is best used as a surprise anyways.
If you lane with him, focus on leveling up Iceball and Blood Boil with just a single point in Consume until late game. If you are going to jungle, reducing the cooldown on Consume is way more important, so I generally go: Q,E,Q,W,Q, R and THEN max Iceball, then max Blood Boil and finish Consume later. This is because level 3 Consume makes doing dragon sufficiently fast, but once you start ganking you really want Iceball as high as possible.
Nunu's problems:
1. You have to pick, AP for damage, tank items for survivability or try to mix them. There are problems and "gaps" in his effectiveness no matter how you build. Go AP, you might get burned down before you get off your ult. Go tanky, you are missing out on a lot of damage. Go hybrid and you aren't epic in either category.
2. His ult is just too dang easy to interrupt or escape. Flash gets you out for free and lots of characters can either blink out naturally, have a speed boosting move to escape the radius or a stun of some kind to interrupt it. You have to position yourself and then time it just right to pull off.
3. Iceball is his ONLY reliable ability in team fights. Consume can't be used on champs, Blood Boil just helps a teammate carry do a little more damage and his ult is completely unreliable. So essentially you throw a snare and then try to position yourself for an ult OR autoattack until you can throw another iceball.
4. His passive is mediocre. He would be way better with some kind damage mitigation.
Nunu is best if you can team him up with some other AoE ults like Galio or Amumu. Otherwise he's just too vulnerable in team fights and his ult generally just ends up being a distraction, rather than a true source of damage. I end up picking him in ranked when no one else on my team is jungling and Amumu isn't available. I play him in Normal games all the time though because I enjoy playing him.
Overall Advice:
1. If your team has some other tanks/tanky champs, go ahead and build him AP with a Banshee's Veil as your only true "tank" item. Haunting Guise is a really good early item for him.
2. Go into some practice games and try jungling with him. Go 1/19/10 for jungling, with one point in improved Smite, then a standard jungler's 10 points in utility for faster leveling and longer duration on neutral buffs. Before you are runed up, start with cloth armor + 5 health pots and do Golem first OR start with Doran's Ring +1 pot and do the small camps and heal before getting the buffs. If you have runes for him, feel free to go Doran's and do Golem first. It's the most efficient path for him.
3. Try to ult from bushes if possible. If not, try to snare with iceball, blood boil to run past them and then pop your ult so they either have to keep running past you to try to escape OR run back into your teammates/towards your tower. Below level 10 this usually results in a kill.
4. Be prepared to get blamed if your team loses. People will call you useless. You need thick skin to play Nunu seriously.
Maybe a little more info than you were looking for, but I hope its helpful!
), I'm just giving you the other side of the story.
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