I'm not playing a lot of nasus anymore,but his E already steals enough CS and pushes hard if you're not used to stack and harass using it. The only times i feel my E should be my main source of damage is when i can't farm (and i don't mean 170- stacks @ 20 min,i mean having almost no stacks at all compared to time). Otherwise having it destroy your enemies armor combined with the OP slow it's enough to Nuke anyone with a Q+Trinity.
I saw a lot of AP nasus during ARUrf week,and all they did was spamming their E without stacking at all. Beaten all of them.
The only nasus i lost against in ARUrf was a fed regular Nasus (Trinity->full tank) who just could destroy our nexus turrets in 3 Q.
How did i play Nasus in ArUrf?
and i got so damn fed (almost 400 stacks @ 20 min and 16/2 or some like that) the i just continued a trolly build

Or well,i would've done it,but the game finished before i could get Death's dance :P
Remember it was Urf tho,so Q every 2 seconds and less,permanent slows and continous DoT+Burst damage from E and Q powered by the armor reduction.
Also,playing an AP nasus would leave you without tankyness,and he has a passive with life steal,so It's not really build around that build.
Edit: Forgot to say 1 thing. Frozen Heart was for AS slow,enemy was AA heavy,also had a vayne,so i did need it a bit. I could've gone for a randuin though,now that i think of it :/