Don't get discouraged if someone tells you that AP nasus sucks, because he only has 1 ability that scales off AP. WRONG. His ultimate and E scales off AP, and is extremely powerful in team fights. His E has a wide radius, and deals decent damage to the entire enemy team. It is also hard to dodge, and is great for harassing in lane. It is also a great farming tool, and with your hextech revolver, its great sustain.

Some useful tips:

REMEMBER that your ultimate increases your casting range! When your ult is activated, you can use the additional range to cast your E a greater distance, and finish off escaping enemies.

FARM WITH YOUR Q - The Q won't do as much without AD, but it can still do decent damage if you have been farming correctly

SPELL VAMP ESSENTIAL - spell vamp is important for AP nasus because he needs the sustain in team fights. AP nasus isn't as tanky as AD nasus, so he makes up for it with Lifesteal and Spell Vamp. Your ultimate and E are great for sustain if you have spell vamp.

EARLY LANING - play safe at first, and farm with your Q. When your E is level 2, use it to harass your enemy while keeping a distance. Your E is great for keeping the enemy health low. Also great for farming. Once you get Catalyst the Protector, mana shouldn't be a problem. Your Ultimate is great for baiting. Some may try to dive you when you are low, so just pop your ult right then.