This kill bonus can be achieved by killing someone with a bullet from the Hail Storm that has bounced at least twice. This kill bonus is most commonly achieved in Dome and Judgement. Some ways to get the achievement are either bouncing it at a stationary enemy from a long distance, or aiming the bullets slightly above 0 degrees at the ground making the bullets bounce more while travelling less along the surface. A third way is to ambush them around a corner. The bullets bounce along the corner, and down to where the enemy are. The bullet will at least bounce twice, giving you the Chill Pill kill bonus.