[Esther]

Or change its focus entirely from a reverse-execute, to a tank-buster. For example:

"A basic attack which beckons even the hardiest targets to an early grave, ignoring a percentage of the target's Defense equal to the percentage of HP you're missing."

That would mean that using it at 250 / 500 HP, against a 50-DEF target, would ignore 50% of that DEF, causing the attack to resolve against a functional 25 DEF, while using it at 10 / 500 HP would ignore 98% of the target's DEF, making the attack 'almost unresistable.' It keeps it in theme with the Ghost class, where it becomes more powerful the closer the Ghost is to death's door, while avoiding silly, balance-ruining things like damage caps and alternate calculations.