We just played against him for the first time last night. I'm still not sure if we played him right; he's SO DIFFERENT from everyone else we've played.
I must have missed the text on the Wagelings that cost the OP the game. Luckily, we only ever got two, and the first was face-down when the second one showed up.
I'm hoping we interpreted him right:
Wed started the game with 4 Conditions out. If I recall, we had the one that lets him win if he's dropped to 0 HP, the one that shuffles our cards into his deck, the one that changes the play order, and the one that lets us discard to flip the Condition. He got a Wageling out, which we quickly dispatched, and we discarded to flip that other one, so he flipped pretty quickly.
Flipping him flips all of his Conditions, if I read him right? So now we could kill him if we could bring him to 0 before he flips again. There were 6 Conditions out when he flipped, so we had ~6 turns to do about 40 damage... assuming we were lucky and he never flipped the "0 HP loses" card. It was pretty tense, as we reshuffled the flipped cards and chose one to flip each round. My wife was seriously tense trying to decide which face-down card to flip to avoid flipping the one that would make us lose if we "killed" him! We managed to avoid it, and we killed him with two face-down cards left.
Assuming we played him right (it took some time to wrap our heads around how he played, and how we were supposed to win if we couldn't KO him!), it was weird, fun, and tense, and difficult not because the cards were unfair, but because it became a puzzle we had to solve. Not being able to kill him by punching him to death made it really feel like a Mr. Mxyzptlk episode, wherein you can't defeat him with conventional means. We had to trick him, play his game, and have a little luck before we could finally put him down.
We had Guise, KNYFE, Setback, and the Naturalist, and it was a lot of fun! Wager Master definitely seems like a once-in-a-blue-moon villain, though. Like, whenever you go to play, choose a villain and then roll a d20, and on a roll of 1 the villain is randomly replaced with Wager Master. :)