100% Plot Armor Kevin. He's powerful enough to make burgalars shit their pants running from his home. No doubt he can just set up lots of extremely clever traps to incapacitate the Wither Storm. He may have to rely on other clever kids like himself in case of emergency, but Kevin could most certainly find a way to destroy WS from the outside.
However, that's just WITH plot armor.
In the case of him not having plot armor, he'd certainly have a difficult time, but he would still likely end up on top with at best a 75% chance.
But, if we count plot armor as his traps as well, then yeah. He's f**ked. He could clone himself 1000 times with absolutely no plot armor and still end up having a 0% chance.
TL;DR: 100% Kevin w/ Plot Armor
75% Without "Immortality" Plot Armor
0% Chance without "Immortality" & Trap Plot Armor
