Nasus is a great champion and i have used him for nearly every game of LoL that i've played. He is the backdoor master, a tower taking, hard hitting, brute with a great AOE and escape option. Play well with Nasus and your team will love you, but get over-confident and you will turn into the enemies' favourite instead!

Nasus wins by attrition; so play it safe and go easy on the mana (or get the blue aura). Nasus is really a carry and needs to farm Siphoning strike & lots of cash before he gets really formidable. Luckily his skillset is perfect for farming, pushing, jungling, harrassing or bruising.

Lots of people say that nasus should solo the top lane - i strongly disagree - Sprit fires' armour debuff makes him the perfect companion to a ranged physical attacker such as Ashe and wither can really help to get an easy early kill. Just so long as the champion you are laning with isn't killing all the minions every wave it is still possible to farm Siphoning strike, and as Nasus is fairly vulnerable early on you will really appreciate the extra help!

If you MUST solo a lane, INSIST on getting the blue aura to allow you to harrass and zone control effectively and keep ranged champions at a distance. If you are soloing a lane against two ranged champions, change lanes; you WILL die. Lots...

In a lane your main duties will be attacking minions and farming Siphoning Strike, only zone controlling and harrassing with Spirit Fire when you have the blue aura.
When you get near an undefended tower, attack it for as long as possible, using Siphoning Strike as much as possible against the tower. Incoming minion waves should be AOE'd to help keep your companion minions alive for longer thus increasing your tower bashing time. When defending your tower, make liberal use of spirit fire but keep some mana handy to use your ulti when the champions get close.

The temptation to push too far with nasus is incredible and must be avoided at all costs. If you feel like you are straying too far from a friendly tower on your own, retreat and jungle untill you have company to help you push in safety, this should let you get the cash and Siphoning Strike stacks you need while not leaving you too vulnerable or an easy target.

Be prepared to be focused by the enemy – you pose a huge threat and they will jump at the chance to gank you, dont give them it! Remember that Siphoning strike has no limit to it's stacks, so keep farming it even late game! the longer the game, the larger your advantage gets! This can make Nasus the ultimate tool in a long, close game.

Cooldown reduction glyphs are a must so you can use your ultimate as often as possible.

Movement speed quintessences are ideal to counter Nasus' kiting weakness.

Attack speed marks boost his damage and regenerative output.

I go for greater seals of endurance.

Alternatively you can go for both greater seals of endurance and quintessanses of endurance for a large health boost (around +330 at lvl 18 which works out to +990 effective health with the build below) but i prefer the movement speed as mobility is very important to nasus.

This build will not make you invincible. However, if you play cautiously you will be incredibly hard to kill, allowing you to deal very scary damage. The health regen from force of nature is buffed by warmogs armour and complimented by nasus' lifesteal, while the resistances offered by both the guardian angel and force of nature let you keep that health. This means any single skill shots champions get off on you will be forgotten about in moments, especially if you are attacking.

The movement speed this build offers will let you keep mobility and allow you to quickly chase down target champions without fear of being kited too far, thus solving nasus's main weakness.

This build is offers high resistances to both magic and physical damage, allowing you the freedom to fight many different champion types including mages, ranged, support and melee. True damage is this build's main weakness as it drops the effective health very quickly.

After getting Guardian angel which item you get next is up to you - if you are against mages, go for force of nature first. If you are against a mixed team, go for warmogs first. Always get phantom dancer last after selling dorans' shield.

Alternative to Ninja tabai is Mercs' treads but i would only consider getting these against a disable-heavy team.

Spirit Fire

– Spirit fire is always the first ability i get. The armour debuff is great at the start and it's a great tool for harrassing, zoning, farming and pushing.

I would advise only using Spirit Fire if you plan on committing to a fight, avoid the temptation to harrass or farm too much with Spirit Fire. Always save at least enough mana to cast each ability once. Getting the blue aura frequently will really help out, but remember to stop harrassing with Spirit Fire the second it runs out. Avoid using low level Spirit Fire attacks against champions with heal or shield abilities, you're just wasting your mana unless you can get minions and another champion in the AOE when casting, then the damage you deal will be worth spending the mana.

Siphoning Strike

– When playing nasus it is imperative that you farm Siphoning Strike. Aim for +100 in the first 20 minutes or so and at least +200/300 by the 40 minute mark but there is no limit to the bonus you can get! This will make you hit like a truck, vital to quickly dropping champions and towers and not bad for getting lots of health back with lifesteal. If you are unlucky enough to be laning with another pusher it can be very hard to get enough stacks on Siphoning Strike so either swap lanes with someone or jungle (when you can) to make up the extra stacks.

At the start, Siphoning Strike is weak and it can be hard to judge when to get the killing blow, but once you have sheen you should notice your stacks shooting up. It is worth mentioning that tower and champion kills with Siphoning Strike also count as a stack, so make sure you try to finish them off with Siphoning Strike. Once Siphoning Strike is at +200/300 you should be able to destroy an undamaged tower in 3 or 4 hits and make any champion think twice about attacking you alone.

Wither

– Wither is a great slow but i only get one rank in this ability until level 14. The great thing about this ability is that it works just fine at level 1, so i keep it there. It has a fairly long cooldown and a relatively high mana cost, so dont use it untill it's really going to give you the edge!

Use wither on the enemy teams main melee or anyone that tries to flee the fight. Wither can be used defensively but try to cast it on a champ with a slowing ability so you can outrun them without giving them a chance to let their team catch you up.

One of the best uses for wither is when attacking a tower because it doesnt draw tower aggro, so cast it on an enemy champion and they will have a hard time defending and most just run away, meaning they leave the tower undefended for a short time - giving you a perfect opportunity to run in for a sneaky SS hit on the tower!

Nasus can do considerable damage, especially if SS has been farmed properly – it is perfect for taking towers as it does insane damage in the late game and is one of the only abilities that can be used on a tower. With the bonus from sheen it is very possible to solo a tower and quickly!

Be wary of being focused in team fights, and don't just charge in! Harrass a bit first with spirit fire before both teams commit to the fight, then pop your ultimate and run on in there trusting to your high health regen, resistances and lifesteal to let you survive long enough to do some real damage.

Nasus has simplistic skills that the enemy can easily predict; so the trick to playing Nasus effectively is to ensure that it doesn't matter that they know what's coming. His predictability can be a great scare tactic too. This seeming weakness is really a strength. Simple skills mean that you dont have to waste time aiming that 50-50 skill shot or wondering what to do next.

Remember that nasus' lifesteal and high damage in the late game can keep you alive much longer than you think, so keep attacking and go for the enemies with the lowest resistances first, but remember to just keep attacking and only flee if you are badly outnumbered!