merchant guild + stampede
Depends a bit on the board, but the plan goes something like this:
1. Get a merchant guild
2. Buy stampede with merchant guild in play
3. Play the horses to hopefully draw the merchant guild, use the coffers to hopefully buy multiple stampedes. Repeat
4. At some point, get another merchant guild (and a source of +actions to play them both), doubling the coffers you get from the stampedes
5. On the turn before you have enough coffers to empty the provinces, get as much +buy as you need
6. On your last turn, you can finally put more than 5 cards in play, since you don't need to stampede anymore
On some boards, there will be a faster engine, but if not, this combo can pretty much always work. It doesn't need trashing or other draw (and often can't even play them much anyway). All it needs is some source of +actions. Even without that, it can still do some things, but it needs the multiple merchant guilds to really explode.