Nasus doesn’t want to fight in the early game. Instead, he wants to focus on farming and stacking his Q. Try to fight him when he is focused on stacking as he may focus on the farm rather than fighting you- which will allow you to gain a health advantage.
Do not push Nasus under his turret as it allows him to farm safely. Instead, freeze the lane towards your side of the map so he has to overextend for farm.
Once Nasus has some levels behind him and has multiple points in his W, he will have good gank set up. You’ll need to play safer if the wave is closer to his side of the map during this time so you don’t die from ganks.
Nasus will be looking to split push throughout the mid-game. He is good at split pushing thanks to his Q.
When Nasus is split pushing, you can use the numbers advantage to start a team fight. While fighting, check the map and be keep an eye on Nasus. Be prepared to disengage if he’s about to take a tower.
If Nasus is grouping for team fights, try to lock him down with CC so he is unable to get on to your carries and kill them with his Q and R.
Nasus is at his weakest in the early game. Try to deny as many stacks as possible so he is unable to scale quickly.
The longer the game goes, the stronger Nasus gets. Make sure you end the game as quickly as possible.
While he will be focusing on farming, once he has his Ultimate R, his kill pressure will increase and so will his survivability. Make sure you’re not too cocky otherwise he can turn the exchange around.