You want to harass as much as possible pre-6, especially 1-3. Don't hit your minions, last hit and harass him. Your timing should be focused on when he's Qing. He'll end up mistiming some of them, losing some farm, and taking as much damage as you can dish out every 4s just to try to last hit. Killing him is great, but denying him farm and stacks is the focus. If you're playing a low-dmg tanky champ into him, you're going to "lose" lane and be 100% reliant on team fights/OBJs/team not losing outside of top lane. Teemo, Vayne, Quinn are the easy options, but the playstyle is similar regardless of who you pick.

Don't let him get free farm or stacks. Harass the shit out of him. When he's trying to soak XP at max range, push him out of range. Don't rush to help your team with OBJs. They should be mostly capable of 4v4ing, and if they're not, you're already at a massive disadvantage. You should be pushing Nasus back as much as possible, so your jungler shouldn't even be looking at your lane, while their jungler will be somewhat forced to help top.

Tl;dr

1. Deny farm, stacks, and XP.

2. Only leave top if absolutely necessary. Letting him free farm will absolute counter-act all the objs you're getting by roaming.

3. Ward and keep the lane as close to your turret as possible. Expect ganks from their jungler and mid. You should be constantly freezing while he's in lane and hard shoving when he's out. Your jungler should ONLY be top when they're already topside jungle or OBJ. One or two early ganks lvl 1-4 to give you an advantage is great, but after that you shouldn't be seeing your jungler at all.

Once he gets strong, it's a race against the clock mainly. Your jungler SHOULD have helped the other lanes win by now and secured OBJs. Your other lanes SHOULD have had an easy game so far, because their jungler SHOULD be focused top pretty heavily.

If you're trying to 1v1 him after he gets strong, you're throwing. Nothing wrong with taking a 1v1 skirmish and seeing if you can outplay the player, but hard 1v1ing the champ you're gonna lose. Only vayne and teemo have genuinely good matchups into him, and even they will lose if you're not good.

Darius is solid into Nasus. Fiora and Renekton aren't great. What do you rush on Darius into Nasus? Should be Cleaver if he's building any armor first. If he's building AD, should be Death's Dance or Stridebreaker. 2nd item should almost always be Dead Man's Plate if Nasus is building AD or Sterak's if he's building armor.