Final Showdown hasn't received much praise from the constructed pros, but this one is definitely an interesting way to exploit Demonslayer Kurtrus. In fact, the cost reduction from both the Questline's steps and reward will make your cards so cheap that you'll be able to easily spam them in combination with Outlander for a huge damage output throughout multiple turns.

Fireshaper and Oops, All Spells! seem to be two of the best Passive Treasures for this build, given that you already run no minions and spells are much easier to discount (Rhonin's Scrying Orb, Khadgar's Scrying Orb and Orb of Revelation are clear examples of what we're implying).

Passive Treasures Chosen

First pool - Fireshaper

Second pool - Oops, All Spells!