I was your opponent in this game. Thank you so much to everyone for commenting. I obviously made a lot of mistakes here:
(1) As has been correctly pointed out, the Herald/Tactician interaction is awful. The reason I went for Herald was that there was only one village in the Black Market (Wandering Minstrel), and also no other good targets for Seaway, and no buys other than a Baron in the Black Market. I made a guesstimate that this particular interaction would occur once in the game, and that one lost turn would be worth it in order to get action-splitting and buys. In retrospect this was probably a mistake. My guesstimate was probably wrong, at least with the number of Heralds I was getting. Has anyone tried to calculate how many herald-induced plays of tactician one can expect in a deck with two tacticians (one of which will tend to be out at the beginning of the turn)?
(2) As has also been correctly pointed out, I made some weak and unnecessary black market buys, including courtyard and haunted woods. I have no excuse for this, this was just an embarrassing misplay
(3) The mistake that felt most pertinent in real time was that I failed to make good use of mission. Several times I failed to play tactician, and I think in some cases I could have recovered by playing tactician in a Mission turn.