Considering when he knew he was dealing with Red Court he brought Flamethrower Shotguns... I think he may have brought Holy/Consecrated Bullets. Kincaid is over 300 years old, so his OW writeup probably rates him lower than he actually is.
He probably used Stealth (Ambush) tactics to take their defenses to Mediocre, Holy Bullets, and probably some unknown powers or stunts.
He also works for the Archive. If anyone is going to know every single trick, loophole, and rules abuse for getting around mystic powers, it's her. Kincaid has always been depicted as crazy-prepared -- as noted by Mal_Luck, he had the shotguns with special rounds in case the vampires cheated (which would have worked nearly as well on a cheating wizard).
I have no doubts that when the call came in for the Archive to work as arbitrator under the accords, he implicitly prepared for treachery from either side. *ESPECIALLY* because one of the two parties was the Denarians and everyone knows just how trustworthy they are...
If we assume that the creature had Supernatural Toughness (suggested for the "Obsidian Statue" on OW-203) and Kincaid could NOT satisfy the catch, then he needs to get approximately 21 shifts to take it out in one shot (3 stress, 4 'extra' boxes, armor: 2, and a total of 12 for consequences). A rifle like the one he was using is already Weapon: 3. He had plenty of time to aim from stealth for a free +2 tag. His target-rich environment stunt *definitely* applied for the +1 to guns. "Technology is the Great Equalizer" and "Protect the Archive" could both be tagged here with little dispute, as well as any stealth-enhancing location tags at the Aquarium. If his Guns skill is Legendary (and I would argue that it is) a good roll against the target's Mediocre defense (ambush shot, as Mal_Luck points out) could produce enough shifts to take it out in a single shot.
And the creature in question still thrashed about, headless, for a while trying to smash Harry.
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So it's possible to take out ONE with a single shot in a prepared ambush, even without special ammunition -- which was how he dropped most of them. The only one that I can't get around is the two-for-one special, unless you count "head blown off" as a really unusual Severe Consequence.
But I think it illustrates how nasty even a pure mortal can be with a gun in this game... if he has time to prepare his shots. None of Kincaid's special powers really came into play during this fight, except possibly ones that enhance his stealth. That example up there will kill a Denarian *without* satisfying the catch. If he was using special ammunition, it becomes a lot easier.
It just occurred to me while writing this up... If a player was trying to do something like this in my game, I would allow the Aim Action for something like "Lined up in my Sights" and then use the free tag to allow the player to do a Spray Attack with a single Rifle bullet from ambush. That doesn't seem very unreasonable to me.