In an Ambassador game, it's always a question whether to add some power soon or go for a clean deck full force.
In general, going for a clean deck full force is better. But here I'm in doubt. Mainly because Mountebank also puts a lot of junk in you opponents deck, and the power cards get really powerful once you get there.
But I do think it is important to chose, and stick to your choice a little bit more then you did.
* If you want to clean out asap (and you did), then why not open double ambassador here? Even if you don't reach $4 on your next run for a village, you'll probably still pick up a haven or two to make clashes less annoying. Having peddler in there may very well make this a viable strategy. You can afford to go really low on money.
* If you don't want to go full force for a clean deck, you need some way to reach $5 quickly. The best plan for this is off course to have incredible luck as your opponent did. After adding an ambassador and a dead village, he managed to ambassador 2 estates, get a haven and a $5. For those of us not using bizarre luck, I think you should sacrifice your mining village on turn 4 to also get a mountebank.
* Accepting your turn 4, as pointed out by dondon, you should keep the curses and get rid of the coppers. I think this one is very important. If your plan is to clean out, any turn you don't draw a curse to block that mountebank is horrible. Once again going very low on money is viable due to peddler.
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Your opponent is really lucky with his early mountebank. Although strategy is an important factor in dominion, a minority of games just are determined by a good/bad luck freak like this one. Better accept it or it gets really frustrating.