Arthas wasn't even that strong 1 on 1. It was his army and power to raise it again and again that made him a ferocious foe. I would put a full scale Azshara invasion with old god backing on the same thread level of the Cataclysm power wise, but shes smarter. She could actually win. She wont of course but you know what I mean.
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My bet is late June 2018. Could just be me being optimistic though.