Brink

To obtain this achievement you have to gun down an enemy bot/player after you have been knocked down (not incapacitated). This happens when you are hit by an explosion or you are taken off of your feet by either an enemy or an allies sliding tackle.

Most assault rifles decimate at close range and most players will obtain this achievement naturally while playing their first few story missions.

A more sure-fire way would be to host a private match with a friend and have him/her slide tackle you while you're firing upon an enemy.

This achievement requires you to 'take down' an enemy whilst knocked down. This description can be misleading - you must incapacitate your enemy. You can be knocked off your feet by a baseball slide, a strong explosion, or a rifle butt (a melee attack from any character using a two-handed weapon) The best weapons to be using are SMGs and Assault rifles, but it's probably best to stick with your favourite weapon, as this is an achievement that should come within the course of natural play.

You will notice that when you are grounded by a baseball slide, your reticule often tracks your attacker very aggressively. This means that although your screen is blurred out by the shocking impact, if your weapon is fully automatic, you will have a good chance of being able to kill your aggressor.

I have also noticed that so long as you don't move the left stick too much, you can stay grounded for as long as you like. If you're particularly hellbent on this achievement, or if it's simply eluding you, you could try staying down and simply acting as though you're camping prone.

You can, of course, boost this in a 1v1 with the help of a compliant friend. (You could also boost the 'Not So Sneaky Now, Are you?' achievement too, while you're at it.)