Doctor's Orders from start to finish, for Leonard McCoy; indeed, from her Trek work, one may confidently deduce that the good Doctor is Duane's all-time favorite.
Followed by the bit in Spock's World with McCoy's epic verbal smackdown of THE ENTIRE PLANET VULCAN?
Oh, and let's not forget afterward, where McCoy informs Spock that his epic, epic speech was "every argument I've ever had with you, rolled into one." To which Spock coolly replies, "Then I would say you won."
The entire crew of the Enterprise gets one in The Wounded Sky, when they travel to the proto-universe next door and, well... to say anything more would be spoiling it.
Naraht get one in The Romulan Way when he tunnels up through the floor of the Romulan Senate and helps McCoy escape under the noses of about a bazillion guards.
From the same novel, Duane's version of the Romulan War is a massive one for the Romulans. It started when the Federation tried a peaceful First Contact, but the Romulans didn't answer. Three years later the Federation returned to try again, and discovered that the Romulans, fearing they were faking to come in peace for a better first strike, had built seven thousands small warships, with which they annihilated the contacting starship and captured the one that had answered her desperate call for rescue. And as the Federation assembled an armada to deal with this apparently unprovoked attack, the Romulans reverse-engineered the warp drive from the captured ship, and countered the Federation superior ships and weapons with huge numbers of small and inferior ships. For twenty five years the Federation continued sending bigger and bigger armadas, and the Romulans smashed them to bits with the concentrate firepower of smaller and inferior but more numerous vessels that were continuously improved with captured Federation tech, until the Federation, advised by the Vulcans (then the new members, according to Duane), decided to cut losses and make peace, giving the Romulans a space where the Federation wouldn't intrude. The Romulans accepted... And started colonizing their new space. Yes, according to Duane the Romulans fought the early Federation to a standstill with the resources of a single solar system. At which point you understand why, in the original series, the Romulans trespassing the Neutral Zone caused a massive Oh Crap for the whole Enterprise...
In this case, Weak, but Skilled beat out Unskilled, but Strong. Our Troi's empathic powers aren't very strong, but she has trained with them extensively for her job as a counselor and to block out the uncontrolled emotional broadcasts of the humans around her (she later develops a form of weaponized empathy in The Battle of Betazed). Mirror Troi is more powerful, but has all the subtlety of a bat to the face, and never even bothered to develop the necessary defense, making her more of a Glass Cannon. Sometimes a scalpel is more effective than a broadsword.
In My Enemy, My Ally Spock and Kirk had a match at 4D chess (a variant of ST 3D chess where you could make timed moves), where our Vulcan was effortlessly winning. Enters McCoy, who takes Kirk's place... And promptly defeats Spock with what was promptly nicknamed kamikaze chess.
Leads to Ael's third moment of awesomeness (the first being breaking free of the Senate's Uriah Gambit against her and recovering her old ship and crew, and the second being causing Kirk an Oh Crap when her ship was identified): when McCoy decided to try and understand what kind of person she was with a game of 4D chess, she defeated him with ease... At her first game. A few minutes after discovering the existence of any form of chess.
Speaking of Ael... She and the Bloodwing had a fair number of scuffles with Kirk and the Enterprise. That's why Kirk was scared when the Bloodwing showed up: the respective number of victories was roughly equal, but Ael's bird of prey was outgunned by far... Except they had just improved the guns with Klingon components.