This guide is based in give Nasus CDR (40% is the cap). The principal source of damage of Nasus is Siphoning Strike and that is the reason why is so important learn to give last hit and do it with Siphoning Strike.
You kill two birds with one shot, because you also learn to farm good and that means more gold to build before and better than your enemies....
The thing I love from Nasus is Wither because sometimes can be really annoying and that will make your enemies rage like hell :) spaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaace

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Pros + Siphoning Strike Spam every 2 seconds. + Wither Spam every 4 seconds. + You are a little bit tanky. + Great damage dealer. + Squishies deleter with Siphoning Strike.

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Cons - Range AD/AS dealers like (ex.Kog maw) are your worst nightmares. - Melee AD/AS are dangerous also. nothing in this line nothing in this line nothing in this line

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• Greater Quintessence of Health: 2,7 x 10 = 27hp at lvl 10. My opinion is that better is be strongh in the begining because in mid/late game items, farm and kills will make the difference.
  
• Greater Quintessence of Movement Speed: These runes are also a very good option, I love mov speed and lately im finding myself better when I use them... so my last opinion is Fortitudes or Swiftness are = great.
  
• greater mark of Desolation: Arpen > Str at mid/late game, also you want your Siphoning Strike to hit hard.
  
• Greater Glyph of Magic Resist: flat magic resist will give you a good amount in the begining to resist magic damage.
  
• Greater Seal of Armor: flat armor will give you a good start armor, in my opinion better than hp. Also in the begining you can notice the effect of the reducing incoming damage.

Start with Doran's Shield, this will give you extra hp, health-regen and armor, good in the begining to hold on the line. Remember that your first mission is last hit minions with Siphoning Strike and second try to get first blood or get a kill if it is posible.
Your primary objetive is get Heart of Gold and Glacial Shroud, these items will be needed later to complete the build and also give you gold while you reach the mid/late game.
Now is time to go for my favorite item Frozen Heart, this item is awesome... dont think it twice and get it!
Don't forget to also buy Mercury's Treads if the enemy has more AP than AD or many CC, otherwise if there is more AD buy Ninja Tabi.
Finish items Shurelya's Battlesong and Randuin's Omen. Now you can start to build Trinity Force. If the game is balanced well then you will be able to finish to build the item, if it is not, then probably will end before you can do it.

For masteries, I had success with 9/21/0. Think that your dmg will come most from your Siphoning Strike and not from autoattacks... so better have more survivality.

Just only one simple thing: LAST HIT TO MINIONS with Siphoning Strike!!!! PLEASE!!!
Look the map, the circle area is your world!... Why?

• You can maximixe your Siphoning Strike quietly.
• Do not run the risk of being ganked.
• You give the chance to your jungle to gank.
• Push? do it when your enemy is dead.

I chose these two spells for this reason:

• Flash: will save you like nothing else, sometimes you need to retreat when the focus is hard. This makes the enemy team lose the target, confuse them to wich target now, and gives you the chance to come back to the battle, helping your team to snare em, etc or just escape to don't give a kill.
• Exhaust: I think is better than ignite in many ways, but the most important is that allows you stop that AD carry that seems to be unstoppable.

You should go solo top so you can improve your Siphoning Strike. In TF you can be one of the initiators, but becarefull, your not a Full-Tank, you can handle some damage, so your team have to know when to act in the right moment.

• Tip: You have always to watch the map and be enough intelligent to know when to leave your lane to go mid for a gank or help budys with problems... one eye in the minimap ALWAYS!

Do you remember all the ganks that you have failed?, all that team fights where 1 or 2 manage to escape? Well.. that's NOT gonna happen again! I promise this to you!
You will be so annoying that you will be hated! they will rage! they will leave! they will flame! and all this only because of your "lamer skill" Wither YES! LAUGH!! MOURN WITH LAUGHTER!!! HAHAHAHAHA!!!!

I hope you like my guide :) I write it just for you and to destroy your enemies and make them cry like noobs!

P.S.(This guide does not contemplate the posibility of being part of a denominated "fail team" or "bunch of f****** noobs").