"Underdog" Achievement.
It would be awarded for a player or team winning with a lower amount of bets than the other players.
The difference of bets would be what counts, so if one player had 2 bets on him and the other player had 4, the underdog would be 2 bets under.
The bronze badge would be awarded for an underdog winning with 1 bet against them, 3 bets for bronze, 6 (or 5) bets for gold, and 10 for platinum.
For 2v2 games, both players would get the achievement if they win.
Maybe another achievement like this in reverse: winning with the higher number of bets. Called Oppressor, or "sure thing" or something.
Anyway, These achievements are pretty cool, but I think a small amount of XP should be awarded for each achievement, like a small amount, but at least something.