300/3900 = 0.07692307692 of your health
As a tank, this means S**t to you. And for god sake, just because you are a tank, does not mean you are unkillable. It means it simply takes longer for you to die. For all we know, from the information you fail to give us, Kat could have full Penetration runes, both of the Penetration items, an abysall scepter and a void staff. Void Staff removes about 105 of your MR, leaving you at 195 MR. A still high amount. The Abysal and pene items lower you down to 145, by removing 50 MR.
484 total damage from Q - 145 MR (Your MR after the Penetration of all her assumed items) meaning you'd be blocking about half her damage(The MR equation is hard, so there isn't a real number I could post). That is not counting if she had runes, or her masteries.
Now here's a fun little fact: Void Staff's efficiency is increased the more MR a target has. So compared to someone who has low MR to someone who has high MR, its gonna be penetrating more for the higher.
My final point here, is just because you built defenses does not mean you automatically win. There is a counter to defenses just like how defenses are a counter to damage. If we had it a one way street, where defenses counter damage without the latter, items that counter defense, then we'd truly be in a tank meta, since there be no way to beat a tank.
