This achievement requires you to 'take down' an enemy with a Mine. Engineers are the only class capable of planting mines. By default, your mines can be selected with LEFT on the Directional Pad.
The description is a little misleading. The achievement wants you to incapacitate an enemy with a mine, not simply knock him to the floor. Nor do you have to outright kill him, simply incapacitate him to the point of requiring a medic (or his own syringe) to get back up.
Not all enemies will be incapacitated by a mine. For example, heavy-build, or even medium-build enemies if they have enough health buffs, can survive a direct mine detonation, and most enemies will survive the explosion if they are sprinting or SMARTing across it.
The two most reliable places to get a kill with your Mines are:
~~ In front of an objective you are defending. If an enemy Operative/Soldier is going to HAVE to stand next to it to complete their objective, you're near guaranteed to get a positive detonation. Also, often a planting/hacking enemy will have taken a little bit of fire on the way, and will be deprived of his health buffs and more likely to die.
~~ You could also try an obvious map chokepoint. If you're noticing a pattern - all the enemies crossing a particular bridge/entering a particular door, simply drop a mine in a likely location, and drop a few rounds into any approaching enemies.
If you'e having trouble online, don't forget that this can be gained playing solo. Simply select Campaign from the main menu, and set your settings to Solo play.