Hello, I have a question regarding readying and the attack step.

Scenario:

I have a Robot Destroyer in play (W/ Doom).
My opponent declares an attack on it with Apocolypse.
We both pass priority on an empty chain.
We then go into the attack substep where all attacks must now be resolved.
He passes priority.
I use robot destroyer's ability to stun one of his 4 drops.

DOES HIS APOCOLYPSE READY?

Thanks in advance :)

Yes, because the defender is no longer present.