Killer Instinct Classic

Pretty simple. There are two methods you can use to get this. With and without a second controller.

The quickest method uses two controllers. While selecting your character, press the A button on second controller to enter Vs mode. Beat the second player. Now have the second player continue and do it 39 more times to unlock the achievement. For an even faster win, press down and medium kick (the B button by default) on the character select screen with both controllers to select the Sky stage. Then simply do a combo with your character to knock them off the edge of the stage for an easy & fast win.

Second method is to play against the computer (Arcade mode). After you win 40 matches, the achievement will unlock. To make this methed easier, set the difficulty to easy.

Bottom line is get 40 wins and achievement will unlock. It doesn't matter where you get the wins from (arcade mode and/or vs mode) as long as you get 40 wins with this character.