Any time any/all of the main characters are put in charge of a force larger than 2 other people.
Will commanding the archers at the end of book 4, The Battle for Skandia, who happen to have previously been Skandian slaves. By the time the Temujai attack, (which is about 2 weeks from when Will is given the archers) they are shooting at the full strength of a battalion of Araluen archers.
Hell, the entire battle for Skandia may be this. The Temujai expect to fight a horde of barbarians who they can curbstomp into the dust in a couple of hours. What they get in return is a well prepared and disciplined fighting force and a battalion of archers, who they rarely ever have to fight. It's mentioned the Temujai are the largest threat to the Western World, and they got beaten by a bunch of barbarians.
The fact that Horace went from a Jerk Jock to being a Master Swordsman, loyal friend, and devious enough to occasionally one-up Halt, of all people, in one of the best examples of Character Development this troper has seen in any work of literature.
Essentially, the heroes were able to train a group of workers who'd never seen a day of battle in their lives to repel an army of experienced Samurai. Using what amounts to Roman military tactics, of all things.
Deparnieux, a local warlord, has imprisoned Halt and Horace. Their solution? Wait for him to lower his guard, then pull an epicBait-and-Switch gambit where it seems as if Horace is preparing to challenge Deparnieux, then have Halt do it himself. Afterwards, Halt does it again, by firing several arrows at Deparnieux. The warlord casually blocks all of his arrows, and when Halt fires a final arrow, he raises his shield to block it...but doesn't notice that it's a special armour-piercing arrow, which, heavier than a normal arrow, dips below his shield and hits him in the chest. He's dead before he hits the ground.
In Book 7, Cassandra proves her Ambadassador status by pointing out to Selethen that since Yusal, not he, was the one holding Erak, she should technically pay him. When he argues that this is a technicality, she gets him to admit that there's actually a substantial reward being offered for Yusal (whom she defeated). Ergo, she does not owe him any money-he owes some to her. She therefore offers to pay 20,000 silver reels to the Bedullin for their help, and another 20,000 to Selethen. When Erak tells her that she's very generous, she looks at him and replies,
Cassandra: No, I'm not. You are. You're the one who's repaying the 40,000 reels to my father, remember?