I'm still haunted by that opening though. Would you guys have gone double Urchin like me or Urchin/Potion like my opponent? I think the double Urchin was a dumb idea and I only won because of strategic roadblock on my opponent's side, but I could be wrong.
Double Urchin all the way...but then you kind of wander away from your plan. The Mercenary idea is to trash anything that gets in your way (Coppers, Estates, Curses, whatever you don't want or need anymore), and keep your opponent with a three-card hand as often as possible. So why ever buy a Potion? Actually, because of the way the game went, you can go through and systematically figure out what you might have done instead.
So: on turn 4, you buy Potion, but could have bought Silver instead. So now pretend "Potion" is "Silver" from here on out.
Thus on Turn 5, you now have $6 in hand (the Silver you could have bought and 4 Copppers) and can buy Laboratory (rather than Familiar).
Turn 6 is meh, but you can buy Silver again I think. Vagrant is mildly helpful to the cause, but you want more draw than that.
Turn 7 in this alternative world starts with Laboratory, Silver, Copper, Cooper, and you end up with another Laboratory.
Etc. With no villages and limited +Buy (and at some point you might want and play a Margrave instead) this isn't going to be a fast game, but good luck to your opponent trying to make hay with Coppersmith and 3 card hands on a continual basis.
