Build and Runes
Don’t let him free farm, his Q will get stronger in late game.
Avoid standing in his Spirit fire as it will shred your armor.
Try to avoid fighting him while his ultimate is active, unless you have the clear advantage.
It’s helpful to take cleanse or buy a Quicksilver Sash to get rid of his Wither as an AD Carry.
Try to avoid pushing your lane against Nasus, as he can free farm under his tower. Use all your skills on Nasus and play aggresively, making him lose HP. Stay close to your own minions with low health, to make Nasus think twice before using Q on them.