Nasus' strength derives from his incredible late game damage when allowed to stack his Siphoning Strike. With Wither to catch out enemies along with Spirit Fire and Fury of the Sands giving him great presence in teamfights, he is a formidable champion, though lacks mobility and hard CC, making him somewhat easy to kite.
Constantly last hitting with Siphoning Strike will have an enormous impact by the end of a game, and will be your primary source of damage.
If you are having difficulty last hitting minions with Siphoning Strike (against strong zoners), request a lane change or a gank.
Wither is best used to shut down auto-attack dependent champions, slowing them to a crawl and reducing their attack speed dramatically.
Choose your target wisely (usually a carry) when casting, as the moderate cooldown is fairly unforgiving since you will only be able to affect one person with it and most casters can shrug off the effects.
With high amounts of cooldown reduction, Nasus can almost continuously maintain Wither on the enemy team (5 sec duration, 6.6 sec cooldown). However, enemy Tenacity can counteract this.
Without any gap-closers or mobility, Nasus relies on his Wither to keep himself from being kited.
Be careful when using Spirit Fire near your tower, as it will push the lane and you will likely miss several minions.
During the laning phase, Spirit Fire will discourage your enemies from farming, but it has a relatively high mana cost and will shove the lane if used too much. Do not spam Spirit Fire during the laning phase. All this will do is waste your mana for when you truly need it. Maintain your Siphoning Strike farm and save your mana pool for when your jungler is ready to gank. That way you can W > E > Q.
Nasus can be itemized in many ways, including a melee DPS, tank, or off-tank bruiser.
Generally speaking, a tank build is considered optimal due to his naturally high late-game damage from Siphoning Strike, which also makes him a priority target for the enemy team.
When choosing summoner spells, either Ghost or Flash have their advantages, with the former granting Nasus more long term mobility to chase enemies down, and the latter to better survive the early game and aggressive ganks.
The Bloodthirster should not be ignored simply because of Nasus' innate abilities. In a more offensive build, The Bloodthirster synergizes well with his passive and Siphoning Strike, causing it to deal more damage early on. The shield will also provide a small defense against harass.
Blade of the Ruined King is a very situational item since it is designed for auto attack heavy champions such as Kog'Maw and Jax, while most of Nasus' damage comes in bursts from Siphoning Strike in a single strike rather than constantly autoattacking your opponent. However, the item's passive and active is useful when facing opponents that benefits from building health such as Volibear, Dr. Mundo, and Maokai.
However, the passive and active do greatly help with his split-pushing abilities and also proc the life-steal both from the item itself and from Soul Eater, adding to his sustain in team-fights. All this makes it a relatively effective late-game choice in a more offensive build.
Randuin's Omen is an excellent armor item as it makes Nasus more difficult to kite, which, despite Wither, is still one of his few weaknesses late game.
When itemizing for defenses on Nasus, prioritize resistances over straight health, as this will make both your innate life-steal and the bonus health from Fury of the Sands more effective. With that said, do not neglect to purchase health entirely, as when you have built some resistances, these will cause it to scale rather well.
Dead Man's Plate, while a strong item on any tank or bruiser, is good for Nasus, allowing him to catch enemies out with the empowered attack's slow and increased mobility when not attacking in addition to its high defensive stats.
Spirit Visage is a great item for Nasus when fighting magic damage heavy opponents, with the increased healing benefiting his innate lifesteal.
Avoid going into a 1v1 battle against a late game Nasus. It's likely that he will have a lot of Siphoning Strike stacks, and built lots of tank items. This combined with his passive, Wither, and Fury of the Sands, means that he will be very difficult to take down alone.
Nasus is not particularly strong in teamfights as Fury of the Sands requires him to be in melee range to utilize its AoE damage and Spirit Fire is not very strong by itself, and he lacks CC other than Wither (which can only target a single enemy champion at once). Therefore, it is possible to ignore him and deal with the rest of his team.
Also, unlike some tanks, Nasus lacks damage reduction or defensive boosts other than the extra health from his ultimate, and the lifesteal from autos and Siphoning Strike are not particularly effective against tanks without armor penetration. Bursting him down before he can utilize his high sustained damage can be viable if you can afford to.
Nasus' strength comes from his ability to farm Siphoning Strike. Try to zone him or deny his chance of last-hitting minions. Pushing him to his tower and making him farm there doesn't help your cause, your just giving him a safe place to farm. Best is to freeze your lane right before your tower, that way if he wants to farm he will have to farm out in the open and be open for easy ganks. Its really hard to shut down a good Nasus
Champions with strong early game such as Darius, Renekton, and Pantheon can be very effective in denying Nasus his farm, but keep in mind that such champions usually fall-off, and once he does get stacks, he will be able to fight you.
You can also ask your jungler to camp Nasus to deny him or even kill him due to his lack of mobility.
Since at times, Nasus will inevitably farm and get to his late game, it may not always be a good idea to use an early game champion to try to deny him. An alternative strategy is to use a tank with reliable CC and waveclear such as Cho'Gath, Maokai, Tahm Kench, or Zac. While they cannot deny farm as easily during the early game, they can outscale him in terms of teamfights - just stack massive amounts of armor, and use their CC to peel him off a carry during the late game. Nasus may attempt to counter this by splitpushing, though. That's where waveclear comes in handy: they can clear waves quickly and delay his splitpushing, buying your team time to force the rest of the enemy to fight.
Avoid using auto-attack reliant champions, as all of them are very vulnerable to Wither. Also, one of Nasus' core items, Frozen Heart, further reduces attack speed, and has armor, making any auto-attack champion a very poor choice unless they manage to kill him early, snowball, and deny his farm. The only exception is Master Yi, due to his immunity to attack speed slows.
Thornmail is a strange, yet effective option. This is because all the damage from Siphoning Strike is reflected by its passive. Not only that, but Thornmail has high armor, mitigating the damage taken from the strike itself, as Nasus rarely purchases armor penetration items.
This item can be effective if your champion has a lot of sustain. For example, Vladimir and champions who can buy lifesteal items, such as Zed, and the aforementioned Master Yi (which can combine his build of crits and lifesteal, with a Last Whisper and Thornmail, to be able to fight a late game Nasus)
Building Tenacity will reduce the duration of Nasus'sWither. However, remember that its maximum slow will be applied quicker than usual due to the shortened duration.
As mentioned above, Nasus has insane 1v1 potential, but poor teamfighting potential. Because of this, most Nasus players will splitpush. Not only will they have a lot of free farm, but his Siphoning Strike can quickly destroy towers, and his dueling potential makes it difficult for enemies to stop them alone, making him a constant threat from across the map. Most champions will loose a 1v1 vs Nasus and will require 2 sometimes 3 other champs, depending how fed he is, in order to kill him.
This can backfire against his team, however. Because Nasus will splitpush most of time, he will have to leave his team with one less member for teamfights. Therefore, he will have to make sure his teammates can win teamfights without his presence, or at least stall and avoid losing towers to the opposing team. Your team can take advantage of this: if you force teamfights against his team and take their towers, and/or splitpush a different lane. This will force Nasus to either stop splitpushing and go help his team (therefore losing farm for his stacks) or risk losing the game without his presence in teamfights.
Nasus does not have strong wave clear aside from Spirit Fire, has no gap closers (Except Ghost), and is a melee champion. As such, top lane champions with long range poke such as Jayce, Nidalee, Ryze, Lulu, Vladimir, Kayle, Lissandra, Gnar, and Kennen can constantly poke him early game and zone. In addition, many of these champions have either some sort of crowd control or escape mechanism to limit Nasus' mobility and make it difficult for Nasus to chase down enemies.
However, when playing a ranged top laner, it is important to note that most AP builds do not have any sustained damage (unless playing someone like Vladimir) and Nasus' late game strength comes from large amount of sustained damage with a single Siphoning Strike. As such, it is important to constantly poke to punish him getting close to a minion to farm and one should be cautious if s/he enters basic attack range as Nasus can punish you using Wither. It is recommended to go for AD sustained damage builds to counteract his lategame damage.