Mill their whole deck

[Balefire Hellkite]

ok, I'm not sure if this works, but here we go...
You cast {Traumatize}, then copy it with a {Reverberate}. Since the Reverberate is an instant, you can put it in the stack first, therefore copying Traumatize instantly and milling 2/2 of their deck.

[Redrighthand]

You'd mill 3/4 of their deck. The first copy of {traumatize} would mill half and the second would mill half of what was left.  Because the milling happens on resolve not on casting.

[mickeven]

you could do  {Keening Stone} after the {Traumatize}

[Balefire Hellkite]

you could do  {Keening Stone} after the {Traumatize}

I've done that before