This achievement is easy to get early in the game. A few missions in, you will have to shoot down a UFO that is larger then the previous ones. Once the mission starts, simply scout a little and you will encounter two Seekers (thanks to DJ Drty Chapstk for the enemies name.) Once they are triggered, just back off and group everyone together. Then just skip turns or hunker down until one of them uncloaks and starts choking one of your soldiers. Then on your next turn simply blast the strangler off. The soldier that is attacked will lose 2 hp when strangled and will not be able to move very far at all after being freed. Your soldier will be out for a few days but you will have, most likely, your first achievement in the game.