...with Shelters. Witch into Rebuild seems pretty clear here.
My opponent gets a 5/2, opens Witch/Estate.
I have 3/4 and get Silver/Feast. Tuns 3 and 4, my Feast becomes a Witch, and I get another feast and a Silver. Going into turn 6, my Feast and Witch are colliding in my hand, and the decks look like this:
At this point, I feel very far behind. I hit $4, and I make what I feel is a pretty bold choice: (with hovel in hand) I buy a Silk Road. What?! Well, tactically, I'm not sure the time is right. But the idea, the strategic thought, is something I am very much on board with.
The concept here is that, though I am very far behind in any case, I'm especially down in the long game. So I make the decision to try to turn the game as short as possible, such that there is more randomness and luck (or less time for the overall quality of his deck to shine through). This should be really losing, but on the other hand, I think everything is really losing, this gives me better random chances, and most importantly, it gives my opponent the chance to make mistakes.
The first of these comes on turn 13, when he buys a Duchy. This is a classic trap in 2 ways: "I'm playing Rebuild, better get Duchies pretty soon" (which is often not true in non-mirrors, Colony games, and various odd scenarios), and "my opponent is playing to end the game quickly, better get points soon in case that happens" (you're often better served by not helping them end things). At that point in the game, he had 3 Rebuild, 1 Estate, 1 Province, and 1 Duchy. If he just focuses on getting those cards up to colony, he's going to have a near-insurmountable lead. He can then finish off by grabbing 1 more green card LATE. As is, the Duchy both slows him down, and it helps me get the piles empty to end the game sooner. This should have been a Rebuild. A similar situation occurs on turn 15, except at that point, he's also locking in one of his own Estates by emptying the LAST Duchy.
He follows this up by grabbing Silvers, in a game where he has very little chance of actually making $8 (though to be fair, actually buying Province would essentially seal the game). By his turn 19, through some luck as well as my mad greening and his missteps, he is actually down by one point, with $6 in hand. He makes a final mistake by buying the last Silk Road, which is only worth 1 point to him. I have one of my Rebuilds, Duchy->Province, and buy the final curse to win.
Let's dig a little as to why this Silk Road is a mistake. There is first of all the general principle of him wanting the game to go long, and this helping me close it out. But specifically here, if I have Rebuild, I can upgrade Duchy into Province or Province into Colony and just win; also, if I have Witch at all, I win immediately - and I am about even money to have at least one of these things happen for me (I also have some very small chance of being able to buy Province). Beyond that, I will probably buy an Estate. If I do that, my one point lead would let him win if: he Rebuilds a Duchy or Province (which requires him both drawing Rebuild and nabbing the correct hit off of its play) OR he has his only Witch AND (in either case) can buy Estate (or, of course, having enough money to buy Province - really unlikely considering he's just seen 3 Silvers). Note that he can't really buy an Estate that turn himself, since even if I were to whiff AGAIN the next turn, one more Estate from me would end the game. But even not buying that Estate, an extra one from me the following turn is worth 6 for me, really putting the squeeze on him (though he would indeed have some chance).
Alternatively, if he doesn't get the Silk Road (instead getting... I'll get to this in a second - for now, let's assume nothing, though that's obviously not strictly best), then what? Ok, well, I can win my next turn if I get Rebuild AND Witch. If I only get Rebuild, turning Estate into the last Silk Road and buying Curse would let me win by a point, but more likely I would upgrade an expensive green card, and then buy the best green card I could afford. If I get only Witch, I win if I can also hit $4. Failing this, I just buy the best green card I can, which gives him a turn to score some points (but he most likely NEEDS to do so right then). Alright, so what should he buy? Estate. It gives me no new wins, scores as many points as Silk Road, doesn't hurt his deck (as he'll never see it again), and there are some scenarios where I can't buy the last Curse, or he can GIVE me the last curse with Witch on his next turn, where it actually turns a loss into a win. Would it have changed the outcome of the game? As it turns out, almost certainly no (I would have been able to buy the last SR instead of curse on my turn 20, which gives him another turn, but he needs to Shanty Town into Witch into a good Rebuild, or ST into two good Rebuilds, which is very very unlikely). And in general, it's very unlikely to. But playing the slim percentages is something you have to do. More to the point, without the earlier too-soon-duchies, I probably would have simply had not enough time to get all the gains I need, he would have been able to get his Rebuilds going to the point where he has a few more points, and then I'd be locked out. These kinds of situations don't come up often, because I put myself into a position where I'm likely to lose, and quickly, which is something people don't like to do. But the volatility turns out to be my best chance, and as the player in the lead, you really need to be vigilant and know which way to turn, because if you aren't precise, you really can open up the door.