I would say that the defender is at the mercy of the attacker, but the attacker wouldn't be able to administer a coup-de-grace if they were being pressed by other combatants, unless they made themselves a similarly open target to take the time to line up a shot through the gaps in the guy's armor and into a vital area. I'd call administering a coup-de-grace a complex action that isn't complete until the attacking character's next action phase. They have to be adjacent to the target, whether they're using a ranged or melee weapon and carefully aim both through the gap in the helpless character's armor and at something that will kill them dead instantly. It's not a coup de grace if you shoot an unconscious man in the ankle, you'd need to vaporize his cerebellum or something. Incoming attacks on the character administering the coup-de-grace wouldn't automatically foil the attempt, they'd need to either move him away from his victim, knock the weapon out of his hand, manually restrain him, kill him, knock him unconscious, stunned or prone, or leave him crippled in agony, such as blasting him with an agonizer. However, the administrator of the coup-de-grace wouldn't be able to use Fray or any melee skill to protect himself, so there's nothing stopping even a relatively weak combatant from opening up on him and shredding him. Generally speaking, stopping in the middle of battle to draw up next to a guy and murder him with great care is a bad idea. On the other hand, there's nothing stopping him from making simple, regular attacks at the guy without lining up a coup-de-grace, and the helpless victim will obviously be unable to use Fray or any melee skills to defend himself.