If Im winning the game with like 10+ team kills, I mess around with builds. Like with Nunu I just build AP instead of tanky and nobody expects the damage. If you level up the Snowball first, it's slow increases per level so that plus the AP damage, it hurts more than expected. Also, his ult, when you have it fully channeled, it does insane damage at lvl 18 without any AP unless they built MR. Stack it with an Abyssal Scepter or something in a baron fight and its a well-won fight.