Because I like all of you so much, here's two achievements for the price of one! They're both on the level "Black Tuesday" of Call of Duty: Modern Warfare 3, and they're pretty easy to get, too.
'Jack the Ripper' states you need to get five knife kills in a row, and will net you 20G on Xbox 360, or a bronze trophy on PlayStation 3. And 'This is My Boomstick' requires you to take out 30 bad guys using your grenade launcher for 20G, or a bronze trophy. Simple, right? Well, let the Achievement Hunters help you out anyway. They're professionals when it comes to stuff like this.