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Katarina is the easiest champion to shut down.
However, she can still melt entire teams very quickly. A good katarina will get 1 kill per fight at the minimum, and force you to either save all your CC for her alone, or risk giving up a penta.
I go glass cannon, so my KDA is horrible, but most of my deaths give me team an ace anyway. I play the suicide Katarina that kills their most fed carry and reduces their entire team to 50% hp or so. I do all this within 1 seconds time. (DFG+E+W is usually 90% of any carry's HP.) If you happen to miss a CC or used it before I got there, I will usually make it a 5 for 0 as long as it's my tanks that initiate.
Katarina is the strongest rolfstomp champion in the game, as she should be. However, she is the easiest to shut down as her downside. It usually depends on the Katarina player.
On a side note: Malphite+Yasuo ults with a Katarina is the strongest thing in the game. Free team fight wins all day every day.