The opening is tricky since the ambassador is vital but the shelters are problematic. Perhaps the first three buys need to be ambassador, lookout, and silver, perhaps in that order. The finish is likely to be a big king's court and governors turn where 6 cost cards are trashed for provinces. The game might look quite different depending on how the players use governors so there isn't just one way to reach that finish.
The soothsayer is situational here. You can use it when you feel that you will win a battle over deck size and can punish the opponent, gaining your own gold at the same time. In a multiplayer game, the decks will take longer to clean up and a soothsayer will look better.