I think I thought of something even slower for the "trash that 1 action card you used to trash everything else", maybe (?), compared to the Death Cart thing.
Put Improve in the Black Market deck.
You must improve whatever card you used to trash all the things into Harvest, into Hireling, then into.. actually we have to stop there, all 3 7$ actions are too helpful. So Improve trashes the Hireling.
How Improve itself gets out of the picture depends. If we use Bard, then I can say, Mining Village is in the kingdom and doesn't cost enough for Bard to get to it, but Improve gets to it. If we use a different card that does not have +coins, I have a clever trick, put "+coins action = treasure" project in the kingdom and trash Improve by buying Mint from the Black Market. How do we get rid of the Mint? You're asking a question out of scope! The challenge said, trash all cards from your deck, not stay deckless for three turns, and during the moment the buy trigger has resolved but before the Mint is gained, all cards are trashed!