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.
YMMV: Zero No Kiseki
Anticlimax Boss: In Zero the second fight with Garcia (as opposed to the rather harder first fight where he's a straight Climax Boss) who is alone and vulnerable to both debuffing and AT Delay, at a time when you have multiple spammable attacks that inflict that status. It Makes Sense in Context as Garcia was fighting against the effects of the Gnosis he was given
To a lesser extent, final boss Joachim is also this, at least by comparison to all the other final bosses in the series. The fact that he has a Post Final Boss segment after you beat him up fairly just adds to the humiliation.
Tiosuke - Tio. This one is actually Randy's in-universe nickname for her which fans have adopted.
Game Breaker: Lloyd and Randy's Combo Craft, Burning Rage. Especially the powered-up Burning Rage II. Better yet, Burning Rage III.
Tio's Zero Field, if activated when CP gauge is full, shields all party members from two attacks. There's a recipe that can refill 95 CP. You get the idea.
In Ao no Kiseki, Wazy Hemisphere hands down, once he rejoins and you get access to his S-Craft Akashic Arm. Easily one shots any boss you come across, and if they're still standing, you need very little in the way of preparing another shot to knock them down hard. All you need to watch is this. It doesn't help that there's a recipe which does two things: refills CP up to 100/200, or take your HP down to 1. And both benefits Wazy, once you figure out the right setup (which takes little effort, except for the winning all POM games).
It's possible to make Burst lasts forever by using Burst Orbs during Burst. Burst Orbs are farm-able. Do the math. Explanation - When in Burst, enemies won't attack your party. So it's possible to win any battle without them attacking you. And that's including the Final Boss, if you can execute Zero Field at the right time.
What happened to the people of Crossbell who didn't have a chance to escape the barrier? And worse yet, Ouroboros was watching.
Shocking Swerve: The revelation that Ian is evil borders between brilliant and this.
That One Boss: Arianrhod, just like her student. What makes this fight especially fun is that If you look at the statistics, you'll assume it's another boss you don't actually have to beat to progress the story. It turns out that you do actually have to drop the boss down to 50% HP before your defeat will let you progress the plot. You're at a disadvantage of 25-30 levels, fighting a boss immune to all status conditions and debuffs and who has an S-Craft that is instant death. Reaching the minimum victory condition is difficult enough, actually winning for the DP bonus and EXP reward requires a very specific strategy, a lot of preparation and some luck.
Campanella as well, thanks to his mix of tricky abilities and his love of Teleport Spam