Here's how it goes:
You have: 2 actions, a Priest, a Trader, a Watchtower, a Copper and a Colony. Play Priest, trashing a copper (+$2), play a Trader, trashing a Colony (+$2), gaining 11 silvers. Reveal watchtower to trash them all (+$22).
I lost a few games to the bot pursuing this strategy, so it's pretty hard to get it to work.