Nasus is actually a pretty weak champion in the grand scheme of things.
His biggest weaknesses are kiting, disengage, and duelists.
Whilst it's true that he can be quite menacing when he's ahead, he lacks the tools to handle multiple champions at a time, long ranged champions, champions who can kite him, or champions who can beat him in a straight up fight.
To be quite honest, in a vacuum, more than half of League's roster has a positive matchup against Nasus in a teamfight. And most of the rest of the roster has the wave-clear to prevent his splitpushing, or they are capable of dueling him.
The classic Ryliandry's setup works very nicely to whittle him down on AP champions, marksman/ AD champions can take BotRK, Black Cleaver, Ghostblade, Mercurial Scimitar, etc. to dampen his effectiveness, and tanks have the option to run Thornmail (which will significantly reduce his damage output from his Q, and also ruins his sustain and healing from his passive, as Nasus does not tend to buy Penetration, meaning Thornmail will actually damage him a significant amount when compared to the way ADCs merely lifesteal through it later in the game), and Spirit Visage.
If Nasus does not have immediate kill pressure on you, consider guerilla warfare, where you attack him, trade some, and then get away before he can critically damage you.This will force him to back off or risk overextending and getting killed.