AP Nunu has never been viable. Trust me, I try to make it work every time I roll Nunu in ARAM.
The problem with Nunu is that while he has quite a bit of base health, his base armor isn't really that good, so surviving long enough to make use of 45% CDR Ice Balls is actually quite difficult if you build full AP.
Runic Echoes is just a bad item though. 60 AP is nothing to a champion that only has 2 AP ratios he cares about (and one of them isn't that high). The MS buff is offset because you didn't buy Boots of Swiftness with that build, should be close to the same move speed. The mana restore isn't relevant, and the burst passive isn't worth spending an entire item slot on. Meanwhile, every aspect of Cinderhulk (Damage over time, flat health, % bonus health) are relevant and valuable.
is a good item, don't get me wrong. I use
because often times, an opponent will Flash away from me and force me to pop my Absolute Zero a second faster than I anticipated, and Chilling Smite finishes them off. I get many kills that way. To be honest, it's just to make up for Nunu's general lack of damage options at range.
Runes - I got these from Meteos's Mobafire Guide to Nunu jungle back in Season 5, they've always been good for me: Attack Speed Reds, Health/Level Yellows, Flat CDR Blues, % MS Quints.
AD Nunu - If you want to kill your opponent with autos, I actually suggest this build:
. Attack speed is what you want to see with the hilarious auto animation, and Nunu's % Max Health on hit damage is a pretty good substitute for AD (It's also magic damage, so it deals more damage than straight AD against targets who don't build resistances, since their natural armor is just higher).