The first thing to notice here is that Ambassador dominates the board.
I am going to go ahead and make a quick note here, that it may look like Possession counters Ambassador. But that isn't really the case - generally, for Possession to work, you need to be able to play it pretty often, and if you skip Ambassador, you're not really going to be in a position to do that - and indeed, you will have trouble even getting to buy Possession. When you add in that the thin-deck Ambassador player can force her opponent to take an Ambassador anyway, the "counter" really looks like it falls apart, at least in the vast majority of circumstances.
Okay, having said that, getting thin with Ambassador tends to just be dominant. This board is a bit tricky, though, because there's not really much to do after you do get your thin deck. Still, you need to go for Ambassador, because there also isn't much to do against it anyway, and there are at least SOME support cards. If there were a good Big Money enabler, a la Vault or Embassy, you could consider skipping Ambassador here. But there isn't.
Okay,s o the next thing to notice is that ways to increase your hand-size are VERY scarce. On board, there is only Wishing Well. And then in the Black Market, you can get Minion, Masquerade, Vagrant, Native Village, Throne Room and Governor. That's it. And it's pretty slim pickings. Some of those are VERY conditional. Being able to increase your hand-size is absolutely massive, especially in Ambassador games, so you will want to go after this hard.
Let's talk a minute about Black Market here. It's possible to go for it - pulling a Masquerade or Governor or Goons can be very, very nice. But I don't like it here, at least early, because you really need to focus on getting to drawing your deck, and Black Market is pretty thorny when it comes to that. Realistically, you aren't going to be able to get to Governor or Goons quickly, so you would really be hoping for Masquerade. And then you have to ask the question, how much better is Masquerade than another Ambassador? It probably is some better. But you've also spent $3 to buy a terminal that doesn't draw anything, and you need to get lucky to rip the Masquerade off the top. Even if you do, having BM in your deck, I am not sure that you're actually ahead of where you would be if you'd just have other 3s instead.
So on a 4/3 opening here, you need to go for Ambassador, and then you have a choice. You could go Fishing Village, which isn't crazy, but I really think it isn't good here. What is it for? The answer has to be another Ambassador, in which case it's mostly just better to get that second Ambassador before the FV. Yes, you can definitely construct shuffles where it would be better, but things are more likely to not collide than to collide, and anyways, getting to play Ambassador on both turn 3 and 4 is a way bigger benefit than the Fishing Village will ever get you. You are much further ahead. So I think you need either that second Ambassador, or Wishing Well. I'm really not sure between the two, but I tend to lean slightly toward the second Ambassador - there's just more upside, I think.
Anyway, on to the actual game:
I get a 5/2 as it turns out, and go in for Ambassador/Hamlet. Hamlet is hardly going to do anything for me here, I figure, but it has to be better than nothing. My opponent opens Ambassador/Ambassador. And I'm already at a significant disadvantage, but the next draws tend to be more important than the opening draws, so I am certainly not out of it.
On turn 5, my opponent buys a Mining Village. I think this is already a pretty significant mistake. He already has a Fishing Village to let him play both Ambassadors. While Mining Village is by no means the worst card available, as a cantrip, and a card that makes it less likely for him to get terminal Ambassador collision, I just don't think it does all that much for his deck, really. I would be much more interested in starting on the Wishing Wells.
Nevertheless, my deck remains slightly worse than his for many turns, as it takes me a long time to get a Fishing Village, which relegates me to getting 2 an awful lot, whereas he can start to build a bit on 3s. By the end of my 15th turn (and I went second), our decks look like this:
However, on his turn 16, he does something which I believe is a bit rash: he buys a curse. This really is how you want to pound someone in many cases, but you really need to be well set up for it first, because you are adding junk to your deck, which, if it doesn't go well, can potentially backfire. Adding a third type of junk to the mix means that you are really making the game about slinging that junk back and forth (though largely, these games already are, to an even greater extent than people realize - perhaps more games on that in the future). You need to make sure you are going to win that fight; otherwise, you should have built more. I am still significantly behind, though, with quite a lot more junk than him.
Between his twenty-first turn and mine, our decks look like this:
The bigger thing to note, though, is that I can't really maintain this off of three wells if I need to 'go big' and contest on expensive cards, since the only way to GET to those expensive cards is by buying more stop card, as those are the only things to produce money. Given that, I need to win the game via the deck-size war. So I chip him back the Curse and the Estate and buy a third Ambassador. On his next turn, he returns the estate and ONE of the two curses he has, and buys a Black Market. I believe this is the key mistake in the game. I believe the best play would be to both return the second Curse as well and buy a third Ambassador of his own. I believe that he effectively can't lose the game at that point - it's not as though I can afford to not return the junk cards, as he could pump me full of coppers, at which point my deck would collapse, and he could win in many different ways. And if I do return junk to him, he can always do it at least as well, if not better. He can add a third Fishing Village and a fourth Ambassador and just pound me into submission. Or he can commence building up slowly at that point. In any case, I would basically have no way to win, given the Wishing Well split, as my deck just can't maintain very many cards. So I would just be stuck treading water, and he would have a long time to pick his spots, very safely. On the other hand, taking Black Market is probably also fine, if he returns both curses as well. He can build up, and he should be able to maintain his deck as fairly clean off of two Ambassadors. He could also keep a curse and gain the third Ambassador, which should be safe and allow him to pound me on deck-size at some point. The big point here is that trying to BOTH build up his economy and half-pound me into submission is biting off more than he can chew. I don't think it's actually doomed to fail (he at least should have some time to change his mind and flip back), but it's at least opening up this hole for me to have an advantage, which he didn't really need to do.
At this point, I want to stress that my opponent in the game is a very VERY good player, and has bested me more often than I have him. I certainly don't want to make it seem like the decision was stupid, or anything like that. I do think it was the wrong call, but it's really hard, really really tricky to make these decisions in the heat of a game, and even the best of players - with Mic certainly being in that small group - can make the wrong call, particularly in such a tricky situation as this.
He pulls a Governor out of the Black Market, but the third Ambassador makes pretty quick work of slamming him with curses, and he is unable to keep up. By turn 26, I have a forced win (though I missed it - it was actually Mic that pointed this out to me), and on turn 27, I empty Curse as the third pile, winning with a score of -2 to -3.