The two solutions above me both work, but if you don't want to have to use the "cheat" method or if you only have one controller then I've got you covered.
First create a guy for career mode then choose a specialty that has a good standing fight. I chose kick boxer so I would be able to kick efficiently as well as punch. Then as you progress in the career keep upgrading you kick/punch speed and left/right hand and leg power.
During fights sometimes if you go for a kick the opponent can and will attempt to grab your leg if he gets a hold of it he will take you down and the achievement won't pop for that match.
I went for their legs first to make them slow so they wouldn't be able to lunge at me and take me down as easily. Then they drop their guard and you can do a nice roundhouse kick to the head and you punch; rinse and repeat.
Eventually you'll be more powerful than an opponent and you will be able to take them out rather quickly (possibly within the first minute; which would unlock the "You got somewhere to be?" achievement for fifty gamer score.
If you have two controllers you can feel free to do the other way.