Well, truth be told... the odds of a party of PCs ever fighting an enemy so much higher level than them that the enemy could do enough damage with a ranged attack to send a single character from Full HP to 0 HP in a single blow... is going to be rare enough that you'll probably never need to pay attention about the rule from the opposite side. It's probably only going come close to occurring when the PCs are at 1st or 2nd level with so little hit points that a one-shot was possible... and at that point do you as the DM need to drop two PCs with a single shot?
They could do it with a Cannon, Sharpshooter, Very Evil Halfling of 18th level Assassin, 1st fighter Archery