attack that can be used only when the char have below 1/3 HP

[AdiDidIt]

How can I make an attack that can be used only when the char have below 1/3 HP?

[STM1993]

There have been a few ideas using DC, but all of them have bugs such as being able to bypass the HP lock. So I don't know.

Two characters I know who have a workable HP lock takes advantage of the fact that MP regenerates faster when your HP is low. Dragonman's secret attack, as well as R-LF2's Justin uses this method. The downside is that you HAVE to use all your mana and if you aren't low HP, you'd just end up wasting all your mana for nothing.

The last possibility is to just use Louis' id 6 which automatically blocks DJA from being used above 1/3 HP, but it also gives you the id's armor and AI.