Only if you play SC5 extremely much you will get this legit, after I got all achievements but this one I was only at 20.000/87.600 meter so really you need to grind this out.
Pick Voldo, cause he is the fastest character, go to quick battle and search for beginner and Europe, a Hilde-style guy comes up eventually called Diamond Joe, looking at his stats he barely attacks and isn't aggresive like most other AI's so it's great to grind this achievement against him :)
Put something a little heavy on the left side of your left joystick so Voldo auto keeps walking left, I used a Yu-Gi-Oh card box but it doesn't matter what you use, for 3 a 4 minutes you will keep walking and never really get hit, press A when the match is over and keep on afk grinding, I did this while watching tv and just looking at my 360 every 5 minutes and got this way quicker than expected :)
Good luck! It's amazing how this has a 5.something TA ratio and so less players have it even a year after the release, it's really not that hard to get this.
I saw the above solution saying 15 hours starting from zero but with Voldo, cause he is so quick and Diamond Joe being a utter nooby AI and barely ever hitting you you'll get this in like 5 a 6 hours :) Just press A twice every 5 minutes and this should come quickly :) No offense to the above solution ofcourse, this is just quicker :)