I destroyed Nasus early game but when mid game came around, he just outhealed my damage. Even though I was ahead in CS and kills, he could outheal me. Soon his Q caught up to my full burst. Like, other than endless ganks is there any outplay to him atm?
vs 
has to be one of the hardest shut downs out there. Start E and literally just auto him as much as you can. His Q isnt up often enough to heal through that kind of harass
First, the general tips:
Try to avoid shoving Nasus under his turret. That makes him very hard to poke and/or kill as most melee champions, leaves you overextended (and wither makes it very easy for his jungler to gank), and limits your jungler's options.
Nasus will typically stack resistances early. In some cases, rushing early penetration (Void Staff, Last Whisper, or Black Cleaver) can help keep your damage relevant against him.
Nasus deals primarily physical damage and he doesn't usually itemize for armor penetration. If you believe you will have to split-push against him for a significant portion of the game, you want to purchase some armor. 
and 
are especially useful when dueling him since they will reduce and reflect damage from his Q respectively.
Shoving to tower is the absolute last thing you want to do against Nasus. It lets him safely free-farm and sustain, limits your trading opportunities, prevents your jungler from ganking him (in most cases), and leaves you vulnerable to ganks (and Wither is great for chasing people down a lane). Diving Nasus can be risky as well since:
Now for your questions.
How do I deny farm?
In my experience, I find that you have to freeze the minion wave and zone Nasus from farm. If at all possible, try and get the minion wave pushing towards your tower, freeze it, and call your jungler up to gank Nasus while he is overextended.
A good Nasus player may know how to handle this situation and break the freeze, but if you can get an advantage against him you can often bully him early. Just make sure he spends as little time under his tower as possible, since they is where he can freely farm and sustain.
Do I roam or stay top?
This greatly depends on the situation. Many counterpicks against Nasus will want to stay in lane with him and deny/bully him for every CS he tries to get. This works quite well when you can do so for an extended period of time, but this isn't always possible (especially when your team is losing). If your champion isn't good for handling Nasus or your team is having issues, you may have to roam to try and snowball other lanes. This can help you end the game early, snowball your team to victory, or get you allies fed enough to help against Nasus. This can be really risky though, since it allows Nasus to freefarm and splitpush.
What champions do I pick into Nasus?
Generally, I find you want some mixture of the following quantities in a champion laning against Nasus:
In addition, it helps if your champion has a strong mid or late game so that they don't fall off in comparison to Nasus. From playing as and against them, I find that the following champions can (or could) do relatively well in lane:
However, you don't always have to counter Nasus solely in lane. There are many champions in other roles who work very well against him.
**Mid laners: **
These champions typically offer high sustained damage and methods of kiting Nasus when he tries to approach their team.
ADCs: 
These ADCs primarily have CC (knockbacks, slows, snares) for kiting Nasus and deal either mixed damage or highly amplified physical damage. Several of them have percent health damage or anti-tank abilities to help kill tanks like Nasus quickly.
Due to the crippling effects of Wither, your ADC should typically have either 
or 
. This can let them remove Wither immediately and kill Nasus before he can reapply it.
Junglers: 
Junglers can be tricky since you typically want someone who is strong and capable of kiting him late game, but you also want someone with strong early pressure to gank Nasus and set him behind.
Nunu can set the enemy jungler behind, relieving pressure from the top lane so they can keep bullying Nasus. During teamfights, he can buff your ADC to increase their damage, chain-slow Nasus to prevent him from reaching anybody, and is too tanky for Nasus to kill.
Master Yi is a special case since he isn't that great against Nasus in teamfights but he can absolutely murder him when they splitpush against one another. Master Yi can deal a lot of mixed damage (Devourer, Blade of the Ruined King, Wuju Style), build somewhat tanky against Nasus (Ninja Tabi, Thornmail), can use Meditate to reduce Q damage, and ignores Wither. He deals far too much damage for Nasus to heal through and deals it too quickly for Nasus to kill him.
Supports: Pretty much the entire support pool can help counter Nasus due to the CC they provide and how good they are at kiting/peeling immobile squishy champions from their carries.
What summoners do I take?
You typically want to take Flash and Teleport. While Ignite gives you some extra kill pressure, you may still lose your level advantage if you have to walk back to lane. In addition, if you don't have teleport than Nasus can freely splitpush and still join fights so you will either lose towers 1v0 or lose teamfights 4v5.
What do I build?
That greatly depends on the champion you are playing. However, I would recommend building your percent-penetration items relatively early in the lane since Nasus will be stacking resistances against you. Afterwards, you may want to get some armor (such as Ninja Tabi, Chain Vest, Seeker's Armguard, or Warden's Mail) or continue building damage.
That's most of the advice that I can give based on my experienced playing against and as Nasus. Hope it helps.
I'm a Nasus main.
You need to play a champion that has early kill pressure on him, but without pushing the wave and allowing him to farm (and Q-stack) under his tower. Teemo is pretty good for this. You need to play the lane well though, because it is very easy to get outscaled as teemo. If you make a mistake, or even if he simply gets a little help from his jungler, and he gets enough gold for early spectre's / merc treads - you're going to be in trouble. Make sure to rush a zhonyas/guise so that you have enough damage to harass him but not die if he all-ins you.
A really good Riven is one of the scariest things for a Nasus player. Just insane oppressive damage while being resourceless--most champions eventually run out of mana and give Nasus a small window to farm, or get punished for Nasus baiting out their abilities.
Honestly though, there are a bunch of champions that do well against Nasus early, but pretty much everyone gets outscaled if the Nasus is competent and plays safely/smartly. The best way to win is not merely by denying him early, but after establishing a nice lead - roaming to other lanes and objectives. Nasus NEEDS to stack for a while before he's strong enough to even consider participating in teamfights...so if you force the action elsewhere, his team will rage at Nasus for making the game a 4v5, because they probably don't realize that they need to play conservatively until Nasus is strong enough to be relevant.
is great against Nasus in lane. Everytime he walks up to Q a minion, q>e>then w when it is available. Rinse and repeat and you'll either kill him or force him out of lane.
Don't take his tower too early or else he'll be able to farm safely at his tier 2 turret, leaving you vulnerable to ganks if you stick around.
Instead of taking the turret, go for turrets in other lanes and put pressure on the enemy. Forcing Nasus to either farming his Q or abandon that to help his team.
Endless sorcerer pretty much said it. However if youre anything like me you might have trouble freezing a lane. Idk your play style but i like to stay between nasus and minions and if he tries to come up for q farm i attack the shit out of him. health pots and more health pots if your champ has no sustain. only last hit minions and never shove minions past the river
Lane swap a morde bot lane up to him
Some good advice here, I hate vsing a nasus. Normally I just try and make plays elsewhere tbh. Which works only if we get enough ahead to get objectives and close out the game early. Probably my least favorite person to lane against mostly cause I like tanky tops.
No... Susan is life.
You let The Cane scale up. scales insanely into the mid and late game. Unless you're a constant split pusher like 
The best way to beat a Nasus is to close the game out really early. If you dont it doesn't matter if he was camped all of laning phase cause he's gonna be a monster in the end.
If you play veigar you can try to out stack him or do the same with nasus. Also you want to not have nasus farm cus thats bad and make sure that u have lots of vision cus if u harras him a lot he will want his jg to prob gank
