Nasus is weak in the early game and will be looking to gain stacks with his . You can abuse this by trading when he goes for stacks- especially if there is a Cannon Minion in lane.
Do not push Nasus under his turret as it allows him to farm safely. Instead freeze the lane or let it push into your turret. Nasus is susceptible to ganks and will need to over extend to farm: which works highly in your favour.
Don't trade with Nasus in his minion wave because he can win the trade with the damage and armor shredder he has through his . If he uses his on you, make sure you get out of it.
Nasus is able to be relevant in teamfights because of his . If his is on cooldown, he will be very weak in team fights. So try and burst him down first so he can’t one shot the ADC.
Nasus does not have any movement abilities. You can exploit that by kiting him or locking him down with CC so he doesn't get to your carries.
Do not leave Nasus alone in a lane while you teamfight. He can take turrets easily and even end the game. Send at least 2 champions to deal with him first.
Nasus gets his first power spike at level 7 if he has Sheen. Do not try to fight him because he wins most of the time.
After Nasus gets Trinity Force, do not fight him with or without your Jungler (unless you’re ahead) because he can 1v2. Wait until he falls off in the late game, or whittle him down first before trying to finish him off.
Nasus becomes powerful after he gets 400 stacks. Do not underestimate him even if he is behind.