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AP Nasus is far and away better than Triforce Nasus and here's why: he has NO AD scaling. His Q, which is why people build Trinity, hits for base AA damage plus farm. Build Lich Bane on him instead and it hits for base AA, plus farm, plus AP. His E and R both scale AP, as well, and with my normal build my ult eats 7% max hp/sec and E hits for ~300 on initial cast. My Q's get up to about 800-900 damage on average by 40min or so, and his E can farm an entire wave with a single cast.
I start boots and 3 pots, go B for a Giant's Belt and rush that into Rylai's. Finish out your Sorc Shoes then build Abyssal and Sunfire (order depending on enemy team, of course). After that you'll want your Sheen item which is Lich Bane, then add a Liandry's Torment for a little extra hp and even more DoT added into your ult/sunfire/spirit fire combo eating people alive just for standing close to you.
I run full tank masteries and level my Q twice early(Q farm is still important, but not quite AS important since you have other forms of damage) then E, then W, hard levelling E after you get the 2 in Q. Generally as soon as you get your ult and your Gian't Belt you'll be able to ult, tower dive, and get an easy kill.