Sivir - the best pusher, midder, flee, great harraser and perfect kser/assist taker.
Sivir is a ranged dps. She is powerful and cheap champion. Actually she is the best pusher champion with great survability against mages. She also can easily defend from spell-based killing blows and ganks.

PROS :
- She can deal physical dmg to enemy champions without gaining minions aggro(ricochet)
- She has the only skill that blooks another skill(spell shield) and if she blocks it it grants her bonus mana.
- Great flee champion. Great dodge chance during flee plus spell shield grants her gank survabiity.
- Sivir is controlling opponent in mid by kiling minions, provoking enemy to use spell on her to gain mana and hard pushing with ult while opponent is out of mid!
- Great farmer - she has a lot of money in early game!
- Blocks most ults! Great against this annoying warwick!
CONS :
- Squishy(but you wont go solo against two on beggining, is it?)
- Bugged spellshield - it wont work on some skills like Karthuss Wall of Pain.
- Mana dependent - you must learn how to use her spellshield!
- Maybe she is recommended, but if you want to do something more than briefly wounding minions follow this build.
- Really, if you want quick game kills go and play Twitch.

We take Ignite and Exhaust:

Ignite is anty heal damaging spell. Great to undertower dive finish.

Exhaust is great chase spell. Combination of haste and this spell gives great effect! Reduces also armor and magic resist upon cripple mastery.
I dont recommend any other spells. You dont need teleport cause you will be too quick for this. No heal. Its counterable spell. No clarity. You will have so much mana from shield.
No revive. It says "I will die soon."
No ghost. You have enought haste. No flash. Maybe good escaping method, but you need to deal good dmg and to not take dmg too. Clavi? Good to check ganks, but you can take ward as well.
I explained the most popular spells. If you want to take other spells, stop reading this guide. Really, I hate people witch want to modyfity one thing in build and say that its "their own mega original superPR(OP)roffesional build."

We take of course offense to make Sivir more deadly and some for defense to make her unhuntable(magic resist and dodge chance plus haste.) Those are standart masteries for Pantheon too!

Marks and Quints: my favourite for most dps - armor penetration! Allows to max dmg by ricochet(crits are allowed only on 1st hit, so crits may not give such great effect.)
Seals: dodge chance - perfect to max out her passive. We can also change it to:
- Hp
- Hp per level
- Armor
- Armor per level
Gylphs: flat magic resist - just to ignore some casters if we cant use shield so quickly. Ewentually we can take magic resist per level.

We max out ricochet 1st. It allows us to both farm and harm our opponent. If you wont get aggro at minions dont attack directly your foe.

Mana shield is 2nd skill(but not to max) just for mana regain and protection form most ults(yay Karth, ty for mana!)

2nd max out is boomerang. Deals nice dmg and is great to finish opponent under turrets and during chases(q+ignite)

Ultimate - great for flee, push, killing, chasing and getting assists(my favourite way is gettinaway with low hp from any fight and ulting on an ally witch will kill enem. Nice assist gain.)

Items are really varied. We take some armor penetration, haste, dmg, crit chance, att speed and lifesteal.while having some kinds of opponent some mr or arm helps too.

1. Longsword plus hp potion - just starting.

Longsword gives us 10 AD at beggining. It's so much in early game. And hp potion for emergency situations.

2. Boots of swiftness for sweetnes.

Allows us to quickly flee from ganks and gank with ease. During late game it shows its adventage over regular boots with 2nd level movement.

3. Ghostblade makes it having sense.

Ghostblade is GREAT for sivir cause of armor penetration,crits if you have to fight directly someone, attack speed and attack + movement speed whenever you want. Good to enpower your flee.

4. Lifesteal hidden in bloodthirster.

Alows you to stand in fight so much longer. Moreover gives the best AD except sword of occult.

5. Bloodrazors if you want to play with tanks.

Bloodrazors is basic item for hight att speed champions - this one will quickly bring down hight-level hp champions.

6. Where does this edge end?

Gives MEGA CRITS. Just insane.

7. It's good cause its black and silver and its cle(a)ver!

Final item. Gives MEGA armor penetration. Now you are battle mistress!

8. Changing items - you can sometimes change ghostblade to other item depending on what you need:

a) Frozen mallet

Really works for solo fight mode. You get also so much hp.

b) Banshee veil

Banshee veil is item that we usually take against casters cause it gives spell negation (now we can ignore two spells!), hp and mr.

c) Another Bloodthirster

Just more ad and lifesteal.

Sivir has two unique skills: spell shield and ricochet.
Spellshield- this is a skill that only Sivir and Nocturne can use(and its a lot better than his spellshield). It will block one spell within 3 seconds.

Akali

Most annoying champion thats anty-squish. Fed Akali is too powerful for Sivir so its a must for Sivir to not feed her. You have to be patient and avoid her twilight shroud.

Annie

Not so much to say: use spellshield on her energized skills.

Ezreal

Just be aware for his Trueshot barrage(especially when he is going ap. it can really scr*w you so hard.

Heimerdinger

Get madreds razors before brutalizer and kill his turrets. Be aware of his micro rockets cause it will harras you easy.

Karthus

Spellshield>Requeim. Seems like Karthus gave you some mana.

Katarina

After lvl 6 she may be hard but if you wont feed her it will go cool.

Swain

Keep spellshield for his snare.

Twisted Fate

You can block his ults sight with spellshield. Meanwhile use it for gold card.