Artistic License – Military: The world in general don't seem to have conventional military doctrine. Military units aren't taught teamwork or shieldwalls. Most everyone fights with a sword instead of using spears or shields. While distinct units like archers and cavalry and swordsmen are mentioned, they don't seem to be being employed properly, e.g. cavalry being trotted out when the enemy flees or is being flanked. The idea of discipline or order amongst the soldiers allowing them to be more effective as a whole is discarded in favor of the idea of individually skilled soldiers and superior numbers being what creates victory. That or the game-changing asset of a rider. And in this and future books, there's no concept of teamwork happening at the squad level, i.e. groups of four or so. When faced with Eragon soldiers don't draw their spears on him, or spread out so they're harder to hit, or lure him with one so the others can flank him, or form a shieldwall for unity, or try combined arms, e.g. some throwing spears while some attack with swords. They simply stand there with swords, make some token individual attacks and defenses, and inevitably die quickly.
There's also no concept of avoidance tactics. Eragon and Saphira are essentially unbeatable, and leadership would've known that from his previous exploits. The best tactic then would be to stay the hell away from him wherever he appears, and do everything you can to negate his advantages. At best, they would give battle before he even arrives, since he's so frequently a game-winning asset. At worst, they should be warding their soldiers against fire so Saphira can't automatically mow them down. Instead they do neither. That's not even getting into the overall goals of leadership and how they conflict with fighting them.
There also doesn't seem to be a clear chain of command. The Varden and Empire seem to fight continuously for a day, with no concept of maneuver or changing parameters. It's simply a huge back-and-forth along the same space with units or specific maneuver across locations not even clarified. It's possible that some are happening and Eragon just isn't being assigned to know about them, but it still paints a very chaotic and wasteful picture.
To their credit however, unlike some other works in this genre the armor is never useless. From beginning to end, Eragon having armor is a defense even against the strongest foes, and there are many times he and other characters might've died except for having it. They got that one right.
Can't Argue with Elves: This book could serve as the trope codifier, as Eragon has many arguments with his teacher and others that near-uniformly end in him admitting they've got a point, and maybe atheism or vegetarianism or just accepting the elves are inherently awesome is the right thing. But it's also subverted, as we see characters like Vanir are exceptionally small-minded and cruel, and that when it comes down to it, Eragon needs to be stronger and smarter and more powerful, and the elves are the ones most likely to give that to him in the quickest amount of time.
The final battle consists of the Varden's vastly outnumbered army - something like 10,000 to 100,000 - deciding to clash with the Empire's in an open field. Even disregarding the Varden's disadvantage, 100,000 is way too large a number to solve this problem, essentially only being good for one battle before having to disband. The Empire would've been better off with 60,000 or so. On the other hand, the Varden would be best off finding a fortified position with water and supplies and drawing out a siege so the Empire would have trouble keeping their numbers in the field and had to send at least some home. Notably, this makes even less sense than the previous book's final battle, as in that case the heroes were defending a fortified position that they took advantage of, the enemy's numbers were much smaller, and they were fighting someone explicitly vicious and stupid enough to throw away his forces' lives for quicker victory.
Eragon and Saphira are probably the worst examples of this. Saphira can breathe fire now, where she couldn't in the last battle. All they'd have to do to win is climb into the sky and strafe the enemy lines, killing hundreds each time. The soldiers aren't warded against fire, and she's a great flier with good armor that can handle any projectiles, with Eragon warding her against any sneaky magical attacks. Even if the enemy improvises some protections on the fly, they can't protect everyone at once.
The Empire gets in on this themselves when their dragon rider is revealed near the end of the battle. He reveals that he was ordered to try to capture Eragon and Saphira, since they need Saphira to breed more dragons. Which means that the soldiers trying to kill her during the battle would've messed up leadership's goals disastrously if they'd succeeded! If they couldn't risk actually hurting them, the Empire should've prioritized giving battle to the Varden before they arrived, and then dispatched their rider as soon as they got there. The author even confirmed the rider would've won even if Eragon and Saphira had been fresh, so they didn't need the army to soften them up! They essentially threw away thousands of soldiers just so their rider could have a more dramatically scary entrance. But hey, maybe that's why Galbatorix is losing.
