This is easiest done on a cavern map. Start a local versus game against a clone using the achievement game settings from the road map. Place your worms up high and the clone worms close to each other but not touching beneath you (you want a large target to make this easier). When it is your turn, select the parachute and jump off the edge. As soon as the parachute deploys, press and select a stick of dynamite. When your falling worm is at a point above the enemy worms, press to drop the dynamite. The achievement will unlock as soon as the first worm dies.