Hello this is why you need to use my build, with the runes and items you will be able to get pentakills.
Heres ya guide, you will jump in with killer insticts on then you throw you blade and use ulti
If you have an good team you will kill them all every time you do that

Role

Katarina's role in a game is simple - she's a ganker/finisher. She CAN carry for her team if she has a brilliant early game with a lot of kills and assists, but don't expect it every game.
More importantly, she CANNOT initiate for the team. You should never be the first person to engage the enemy team in a pitched 5v5 battle ala Malphite, your job instead is to sneak in unawares and quickly take out priority targets.

As the game progresses, you'll steadily gain more team presence as your ultimate increases dramatically in power, until eventually you'll be hitting as many as 3 enemies at once for tons of damage multiple times per battle.

Due to her fragile HP, Katarina should never be expected to tank for the team. Even so, you should expect to be targeted first in every battle due to how much of a nuisance you are. We will have to choose our items carefully so that we can survive the initial assault, as well as use Shunpo and Flash to back off if the heat gets too much.

Now about the runes.
AP runes: you will get an amacing early game with you crazed AP in low lvl. If you got all the runes you will get about 24-25 AP. Thats why you need to buy theese runes first.

Magic penetration: You Magics will smash trough theire Magic resistance, thats why you need to buy you Magic penetration after you AP

Cooldown reduction: The reason why you need to buy 1 Cooldown reduction rune is to get a little bit of the timer of you ulti with you rune and you passive you will be able to spam you ulti everytime you get kills

IF you dont buy an Hextech Gunblade as Katarina its pretty fail. You will get no Lifesteal/Spellvamp.

If you still buy AD as Katarina you need to now 1 little thing thats make a big diffrence.
AD has been nerfed as Katarina. Her AP is boosting everything, and her AD is boosting Ultimate and Bouncing Blade.
You Shunpo makes alot of dmg with AP

With Exhaust you will easly kill champs like "Tryndamere and teemo"
And if people is hunting you, you will escape almost every time

Skills


Q - Bouncing Blade - Katarina throws a dagger that bounces from enemy to enemy dealing her weapon damage + bonus magic damage. Subsequent hits do 15% reduced damage. The number of enemies hit and the magic damage scales with skill level.

This is the first of your damage spells. It has multiple uses - harassing an enemy in lane, last-hitting a creep, farming (with a DPS build) and most importantly - finishing off an enemy.
This skill scales with weapon damage, and is unaffected by AP.
This has the second shortest cooldown of her spells.


W - Killer Instincts -

Passive: Katarina's attack speed is increased each time she consecutively hits the same target. Active: Katarina's next basic spell has a bonus effect. Bouncing Blade does full damage to every target hit and reduces incoming healing to the target, or after using Shunpo Katarina's dodge chance is increased for 3 seconds.

A strange skill. The passive ability is useful for DPS Katarina builds, but be warned - the bonus attack speed only lasts for 5 seconds. The best use for this is when taking down towers.
The W + Q active combo is generally less useful than the other, and is only good if you have good weapon damage. The healing reduction is very situational, though it will be good vs. a heal-heavy line-up.
The W + E active combo can be quite useful in a team battle, as it gives you significant dodge (70% at level 5) for 3 seconds. This could give you the precious few seconds it takes for her ultimate to kill someone, allowing you to safely Shunpo away with another W + E combo.
This has the second longest cooldown of her spells, so be sure you use it at the right time.


E - Shunpo - Katarina instantly moves to her target location. If the target is an enemy, she deals damage.

Katarina's signature skill. You will be using this one more than any other. From last-hitting in lane, to harassing enemies and positioning yourself to cast your ultimate in the middle of a team battle - this skill does it all. And of course, it's a beast of a chasing ability - especially when paired with a 40% slow from Rylai's Scepter.
Most importantly - this spell scales with a 1:1 AP ratio, and has the lowest cooldown of all her spells (3seconds at level 5 with 40% cooldown reduction!). Imagine a nuke that does 300+ damage every 3 seconds and positions you right next to an enemy so you can hit him...


R - Death Lotus - Katarina goes into a state of extreme focus, throwing daggers with unrivaled speed at one enemy champion per level of the spell. Channeling.

Katarina's most controversial spell. This spell has a very low range, is channeling (which means it can be stopped by a stun or an ability that forcibly moves your character) and seemingly does low damage.
So does it actually suck?

Hell, no!

At 40 seconds, the cooldown on this ultimate is incredibly low for the kind of spell it is. This is further augmented by her passive, which allow you to instantly cast the ultimate again if two enemies you've hit in the last 15 seconds die before the ultimate runs out.
This is exactly why Katarina is so powerful in team battles.
It's even powerful in the laning stage, and I'll describe why shortly.
The ultimate scales at a 0.15 AP ratio which might seem low at first, but is actually more than enough for our purposes.


(Passive) - Voracity - Every time a champion Katarina has damaged in the last 15 seconds dies, she gains 50 gold and her cooldowns are reduced by 20 seconds.