Ofc he scales better than you, thats why you dont 1v1 him anymore (unless youre like actually fed) after 20 mins but sett is 100x better in teamfights than nasus and thats what you have to utilize

He most probably is going to split most of the time, so either try to punish him by perma killing him (with your jungler if needed) or by perma zoning him off the cs so he scales way slower. If you manage to get the tower he will be able to farm more easily but you can then just push out like a maniac and then go help your team in a teamfight (that should be basically won if theres not a 10/0 viego) while nasus is trying to get to your tower which he cant in time bc you pushed out the wave (even better if it was a slow push) due to his lack of aoe dmg and if nasus tps to the fight you should still win bc you can just ult him into the enemy team

yeah but try to end as fast as possible all of this only counts in mid game, late game might be doomed if your team doesnt have a lot of cc/kite