Shoot the Cyberdisc to kill it when its one level above the ground, it drops down to ground level, hits the ground and becomes a corpse, THEN the explosion goes off where it was just hovering: ie: at one level off the ground.
Note: you may have difficulty getting an intact corpse because of its explosive nature. The primary anti-gravity system is too badly damaged to gain any understanding of its functioning. The Cyberdisc is well shielded and is particularly good at withstanding explosive ammunition.
