This is to help people that know that they're doing well enough to get the achievement, but it's just not popping after many attemtps:
So I started thinking WAAAAYYY back to early 360 days, and was trying to remember what I would do for achievements that just wouldn't pop for whatever reasons after a ton of close but no cigar attempts. I got in there and cleared the hard drive cache then started another NEW GAME. To do this go to the 360 home screen and go to the settings tab. Select storage and go to the main system hard drive (assuming that your main save is on the cloud which it should be because you never know what may happen), and then hit Y to get the option to clear the cache then start the game up again. By this point I've got it perfect where I kill at least 9/10 consistently every run if not hit all 10. After doing that first run on JP version achievement unlocked. Hooked up the NTSC 360 I was playin on, and cleared the hard drive cache on that one too. First run on NA version achievement unlocked (hit 8/10). If you're having trouble unlocking it even though you know you're hitting at least 8 people consistently just give this a try.