When Ramza and Izlude face Chaos, the first Lucavi seen in the story, screw the fact that they're alone and have never seen such a thing before, because they take - him - down - with nothing but their teamwork together and skill. Meaning that they proving from an early point that a) the Lucavi are not invincible; and b) anyone trained under a Beoulve is destined to become an inherent badass.
Meanwhile Queklain deserves credit for being able to hold off Mel, Alma and Izlude together, since he only really went down when Valmafra showed up to help. Given he was originally a Starter Villain in canon, this shows the Lucavi aren't being treated as a joke even in the early stages of the story.
Ramza's Duel Boss fights with Gafgarion and Wiegraf - not only for being good achievements for the youthful hero (defeating much older and more experienced adversaries), but also for the author, who deserves praise for creating such effective battle sequences.
In detail, Ramza fights Gaff twice, first time on his own, second to try out his new Dark Sword abilities, and both times he won, sparing Gaff's life for favors. When he fought Wiegraf, he made it look real easy at first. Even when Wiegraf summoned his sword arts, Ramza neither broke in stride or faltered from the attack, as if he never felt it, and continued walking until attacking close ranged with "murder in his eyes".It was then the White Knight knew how screwed he was.
In Chapter 23, Zalbag and Agrias's fight with Zeromus/ Barinten. It's probably the best-written fight scene up to that point, and easily one of the best battles in the entire story in general. It took a villain from the original game who was simply a despicable and abhorrent individual, and turned him into a potent threat. Followed by:
Chapter 28, where Zalbag and the Lionsguard face off together against Mateus, who tops Zeromus in terms of potency in battle. Besides that, it's made even better by the fact that Alicia and Lavian's actions are what bring about his defeat in the end. Of course, come Chapter 29, this makes their Heroic Sacrifice via You Shall Not Pass hit even harder.
First is Valmafra's magical duel with Kletian, not only for the sheer epicness described in the battle but also because she takes him down using other abilities he didn't teach her, taking him by surprise despite his raw magical power overwhelming hers.
Kletian: It's a pity, however, as it seems you didn't learn enough.
Valmafra: Not everything I learned came from you. I'm an assassin, remember?
Second is Wiegraf's fight with the Lucavi demon Velius. Even the offer of power and losing an arm didn't deter his determination and morals unlike in canon, and while he loses, he does save Ovelia's life.
The entirety of Chapter 36, the "Airship Battle" chapter, which delivers just as the author has been promising.