So, I've noticed how only 3 jobs have OH-KO skills, so I decided to make some up for the rest of them.
Warrior - Heart Pierce (single target)
Pierce through a single enemy's heart with your sword, instantly killing them. Doesn't always work.
Mage - Death Hex (single target) (has a similar animation to the Fiend's 999 damage attack)
Cast a spell to kill an enemy. Doesn't always work. (Magic)
Thief - Lifesteal Dagger (all targets) (The success message for each enemy appears one at a time instead of all at once.)
Make your dagger so sharp that it has a chance to slice off an enemy's head.
Pop Star - Lament (all targets) (This move is signified by the Pop Star holding out their mic and tilting their head downwards with a dejected facial expression. They have this same expression while using the move but their eyes are closed. To add to the effect, the colors around them fade while they are singing.)
Sing an unusually depressing song that has a chance to drain all enemies' wills to live.
Cat - Claws of Death (single target)
Scratch an enemy so deep that they may not survive. Doesn't always work.
Scientist - Virus (single target) (The loading circle is dark purple when using this skill)
Cause fatal errors in the code of an enemy. Doesn't always work. (Magic)
Tank - Nuke (wide target) (Even if you miss the main target, the ones to the sides still have a chance to be hit.)
Shoot a bomb out of your cannon that has a chance to kill an enemy and the ones that surround it.
Princess - Execution (all targets)
Order for all monsters to be executed, killing any who don't dare disobey.
Flower - Wilt (single target) (If it works, you get restored to full HP and MP.)
Drain an enemy of vital nutrients, causing them to shrivel up. Doesn't always work. (Magic)
Vampire - Blood Drain (single target) (If it works, you get restored to full HP)
Sink your fangs so deep into an enemy that they completely bleed out. Doesn't always work.
Elf - Lifesteal Arrow (single target)
Shoot an enemy with an arrow enchanted by cursed magic to steal their lifeforce. Doesn't always work. (Magic)
I know some of the descriptions are a little dark for MiiTopia, but I'm sure you'll enjoy them nonetheless.