Gears of War

This achievement speaks for itself - simply kill one hundred enemies with a headshot in ranked matches.

Many weapons can perform a headshot, such as the Snub Pistol, the Longshot and the Gnasher Shotgun. The best, surefire way to get a headshot is with the Longshot, and simply picking off the head of an exposed enemy. However, random headshots can be performed with the Gnasher Shotgun, and with the Snub Pistol when aiming at the head.

This isn't really a solution, more some advice for people trying to unlock the various kill related achievements as quickly as possible - at the end of every boosting session you should play a quick one round local match against a second controller before quitting to dashboard or turning off your xbox. I've done this and unlocked Seriously yesterday pretty much bang on 10000 kills. I've also unlocked all the weapon kill achievements after 100 kills.

I think what's going on is that sometimes, for whatever reason, the progress you make in the last match of a session isn't added to the various achievement counters. I first noticed this when boosting the 'Win 20 matches on map X' achievements - if I won 20 matches in one session the achievement would pop, but if I spread them out across multiple sessions I'd have to play 21, 22, sometimes 23 matches depending on how many sessions it took.

Of course, it's possible I've just been really, really lucky - there's no way I can prove this works. If people start trying this and it doesn't work out let me know and I'll take this solution down.

This achievement, like many other online weapons achievements for this game, is glitched. I personally unlocked it at around 140 but have seen some people go above 200 just to unlock it.

You can get this achievement with most weapons such as the gnasher shotgun, longshot, snub pistol, lancer, etc. For some unkown reason some people cannot get this achievement with the longshot while others can't get it with the pistol. My suggestion would be to try and get it with the longshot achievement or with the pistol achievement as you should get it with one of them.

If you only count the kills that you get that are not in games that either lagged out or had people leave then the final kill count might be closer to 100.