Honestly I would stick the Abyssal Mask in there somewhere, it will mostly solve your mr problems. Also consider MR per level glyphs, the cooldowns are nice, but only if the enemy AP doesn't start nuking you.

If you want take a look at my Godzilla Cho'gath guide. I tried to do something similar to what you did here. So far people have been telling me that the build works amazing and it has yielded great results for me.

I am still going to vote it up since I still feel its a pretty good idea. I would swap gunblade for abyssal, it will bring your build cost down, make you an off tank, and keep your magic pen from trailing off much. While the gunblade is nice Nunu definitely doesn't need it. You can consume for health and the attack damage boost isn't enough to break anyones back.

I like the guide but I don't get the Gunblade.. wouldn't a Lichbane be much better for an AP bruiser..? Masteries are a little unclear. What exactly does the Tower dmg mastery do for you and why no movespeed? nice Guide

TY ;D its not even close to being finished so check back in a little while and I'll have added a bunch of stuff to make it easier to read and a bit more in depthw

Nice guide, liked :D

Just added a section as to why I take heal over the aforementioned spells. Tenacity does not cancel any disruptions only shortens them, so sadly this wont do anything for Nunu's ult, which is why it is a very 'Risk vs Reward' spell. To utilize your ult late game you will need help from your teammates to pull it off, I will describe how in a future update. Also as a pusher, my build relies more on you buffing yourself and a teammate and pushing the enemy team out of lane to secure a turret, different than a way Nasus pushes but just as effective.

I wont rate this guide yet cuz its really not a full guide.
1. An ap pusher should have a lichbane, The lichbane procs on every spell u use due to ur ap so does extra dmg to ur ap which is how some ap are really great pushers.
2. u will need mercury treads and not sorceres Because for ur ult ur gonna get disable the whole time wen u ult so u need Tenacity.
3. wen u ult as i said ur gonna get disabled then why not egt bancheis .
4. As an ap nunu u will need Ignite or cleanse and not heal

Please comment on what you would like to see added/if you think there is a change that should be made, all comments welcome! This guide will be under construction for a good while :)