Not much to comment on in the general case here. Apothecary engines are the obvious best thing, with Stonemason, Warehouse, and Mystic all as quite good support. We mirror, with me going second.
I want to join the game after the Apothecaries ran out, which happened on my opponents seventh(!) turn:
But at this point, I am already in Endgame mode. Apothecaries are an empty pile. Four Warehouses are gone, and Stonemasons are both lowered and very easy to run. The game can't end right away, but jockeying for position is something you already need to think about. On my next turn, I draw my deck (sans one estate). I turn my potion into a pair of Warehouses, for improved reliability (and the key thing here is to discard Coppers - later Apothecaries become AMAZING). And then I have 9 - I could buy a Province, and that's probably ok, but I feel like it doesn't position me well for a long game. And while I am already considering how piling out can happen here, it isn't going to happen just yet, so I do need to be prepared for a longer game. For this reason, I take Stonemason for a Festival and a Mystic. The Mystic is a significantly better card for the deck, as it's going to hit a huge percentage of the time, but the Festival gives me access to an extra buy, an extra Stonemason play, and TONS more pile control. It isn't terribly common that two different cards off a Stonemason overpay is the correct way to go, but I think it's definitely the right call here, as the festival is really needed right away, and the Mystic is a clear step better after that.
My opponent counters with an identical turn.
And then I have a complete dud, producing only $3. I buy an Estate. This is actually absolutely critical. The game can already end at any time. Overpaying for Stonemason to get Warehouses (only 2 left) and then again for Stonemasons themselves (4 left right now, down to 3 after the Warehouse overpay) will end the game. So the point is actually pretty important to hold off pressure from my opponent doing this (he could potentially still win this turn, but it would require a near-perfect draw to empty AND score 2+ points), as well as to be able to more easily threaten this myself. On turn 9, my opponent builds more, getting another Mystic and a Cellar. It's possible he should try to score some points, in order to block me, especially considering that my previous-turn dud means I'm pretty likely to go off this turn. He's in a very tough spot, though, because scoring points is going to continue to hurt his long-game chances.
In any case, I do have the monster hand I need, and finish it out.