Since I hate boring decks like this one (and CDPR directly stated they never meant to have a "mill archetype" and will make sure it won't be viable in the future), I will offer my humble contribution.
I kept count from this hotfix's release, in ladder my score against this deck is 6-1-0 with a 25-cards deck.
The key is to pass as soon as Emhyr took Avallac'h back to the hand, after having played as much high-tempo as possible from the beginning.
His Emhyr low-tempo is necessary to set up the mill, but removes power from the opponent's board, and as long as you have passed he won't be able to use any of his milling tools.
Make sure he'll have to fill the gap with sub-optimal play, so you'll get 2+ CA and mitigate most of his later work.
Ofc, kill Avallac'h (or Artefact him) asap if there is the chance, sometimes that will slow the enemy enough to cripple the mill.
Do not use spies or low-tempo play unless you know the mill has been stopped, play aggressively and aim for the 2-0, pass when it's strategic.
If NR or NG, use Nenneke or Assire with high-power Bronze units.
If using weather, make sure the enemy board is coated (esp. Siege row) before passing.
If you have access to ticking damage/buffers (Archespore, Trebuchets, Drummers, Yencon, Triss:Butt) use them all before passing, he won't keep up the pace.