Hi this is my first ever build and i decided to start off with a nasus build as he is my fav champ. He may be quite weak early game but at late game he can almost take on a full team!!

Pros

Insane late game damage if farmed correctly
Easy to play
Quite a bulky champ
Is able to solo lane

Cons

No potshot abilities
Vulnerable to long ranged champs like caitlyn and nidalee
weak early game
Often targeted in teamfights

Farming as nasus massively affects your damaged output in late game. Try to last hit minions with siphoning strike. Using spirit fire over the minion group will make it easier to get off siphon strikes as it deals lots of damage to enemy champions who stand in it making you less likely to be potshotted.

Sheen
Sheen is a good item for nasus as it massively increases your damage output, try to use whither or spirit fire before using siphoning strike to increase the dame that you deal.

Berserker greaves
This are the boots of choice when playing as nasus, the extra attack speed contributes well with your passive, Howeever if you are against a speedy team then invest in boots of swiftness instead.

Trinity force
This is the best item for nasus imo, it increases his damage output all round and cotributes well to the team as you can slow them without using wither or exaust.

The Bloodthirster
This item is great for you damage output and also massively increases your survivablity provided that you are attacking an enemy.

Infinity edge
In late game you start to crit a lot with siphoning strike and adding this item can assure that your crits do over 1000 damage.

Phantom Dancer
This item scales well with Bloodthirster and infinity edge because the more times you crit the more you will heal from lifesteal.

Hextech Gunblade
It is rare that you would get to the stage of getting this item, but imo it is very good item for teamfights as it has got a slow and makes u heal insanely quick. An alternative to this could be tiamat

Please try out this build as i think this will solve the problem of bad nasus players :)