These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Rufus after a certain level starts gaining a ton of hp, atk and fairly decent levels of defense. For his atk, however, he probably should have more defense, making more of a sponge than a Lightning Bruiser.
If you manage to get it, give him Godspeed and watch as he utterly annihilates entire maps in one to two turns.
Rose and Colet are two of the better characters in the game due to how dodgy they are. Sadly, they can only take a hit or two before getting taken out.
Colet takes it one step further with two, three and four hits. Each one counts as a physical hit, which drops burning auras onto the ground or onto characters. Since the burning auras stack on top of each other, you do it when Rufus is standing by or behind an enemy and suddenly Rufus can one-shot kill anything within range of his axe.
Jeanne when she returns to the party becomes this late in the game with up to five different transformations, everyone else gets only 3 if they can transform at all. especially when you realize that she gets some of the best armor and weapons in the game and is likely among the highest leveled characters in your party at any given time.
Counter and Counter 2 is stupid broken when it's combined with Adroit, which means you will always face your foe no matter which angle he or she is attacking from and have a 30% chance of countering the attack.
Two Rounds and Three Rounds, when given to Marcel, should be almost considered a war crime. He'll start shooting the enemy two to three times with whatever skill he used, whether normal or special (so long as he has the magic for it).
Sky Dart, a skill that can hit from anywhere on the map, and has a low MP cost allowing it to be used continuously. Especially powerful due to you gaining not one, but two characters who are able to use it.
Characters that can transform can be considered this. Not only do they get a stat buff, they get a health buff, full recovery on their health and automatic access, while transformed, to the Godspeed skill, which, if they kill an enemy, lets them move again right away. In all fairness, this bit is invoked.
That One Boss: Tiamat. Also every enemy in the Colosseum II and III after Round 8, but Luther takes the proverbial cake and Counter 2s it into oblivion.)
That One Level: The Escort Mission where you have to escort the Dauphin to the church. He only goes in a straight line, and walks into the range of arrows This unfortunately makes this level quite annoying, as no amount of leveling will help you get past this.
Until you realize you can usually One-Hit Kill said archers with your own archer by using the Sky Dart skill.