This is what I've run the last ~2 years in Gauntlet (with some updates). I'm averaging 1 loss every ~400 games. Knowing all the AI decks is integral to that success rate and eventually I plan to add an extensive write-up on optimal cards in your opening hand against each AI deck.
The most "flex" slot are the 3 copies of
Xenan Lifespeaker, which could be changed to 2 or 1. With the drop of Battle Lines there has been a complete overhaul of gauntlet boss decks and Lifespeaker was a vital card against the worst matchup (double dmg). I have only done 8 runs so far in Battle Lines, but I have yet to lose a game with this configuration of cards.
I could/should do a full breakdown of why these cards are in the deck... but I don't have the time. I will say that I often see people change cards from this list, especially market cards, especially Incarnus. Remove Incarnus at your own risk - the card makes the games up to the boss so much smoother and faster. There are certain play patterns that you may need to learn, but one tip I can give is that I haven't found a card in the game with higher threat level to the AI than Syl (for its targeted removal). Also the AI has no idea how decay works.