I had a way to bounce his big artifact creature back to his hand after he brought it out with Tinker. Karn then got mana-screwed, didn’t draw any of his Mox cards (free artifact mana). In the meantime, I got up to seven mana and cast Razorfield Thresher. The best part was that the Thresher was only in my deck as a total joke. It was a random card in one of the pre-constructed decks that the game offered, and it was just begging to be played. Since Karn was able to literally do nothing for several turns, I simply beat him in the face for 6 damage for a few turns. I then proceeded to win target game.
And that, my friends, is why Razorfield Thresher is awesome.