W alone doesn't kill the creeps but it does help clear them, i find that one w(maxed) and one e is enough to kill most caster minions, but only those that you can hit. The problem however is when the push is accompanied with more one creepwave. And using your ulti to clear a creepwave doesn't seem to be very efficient way of doing it, as it leaves you with one less utility when a team fight breaks out.
The most common situation i find is that one tower is being pushed, and i just happen to be the only one in immediate vicinity to respond. I've tried building ap items on her but found its not worth the effort to do so for her w and e and instead concentrate on tank items.
I do however notice that if i reached the tower before the enemy, they are more apprehensive to attack it. However if they're already hitting the tower when i arrive, they just continue on doing so. since i do not do enough damage, or they think they can survive a 1.5 stun from my q and another 1.5 stun from my ulti (also at least wasting it just to stop a tower push).