Creating an event in battle - need assistance

[Gespenst]

I want to create an event in battle similar to Deus boss fight in Xenogears where if the boss gets damaged with any kind of attack - he'll instantly react by completely getting healed up to full. The way to defeat him was to allow him to use his special skill where it halves the current HP of all characters on the field including himself, so the strategy was to allow him to lower his HP enough that you can kill him within 1 attack so that he won't instantly full heal.

The HP Halving can be done with a gravity script I have but I just need help on recreating this similar battle event.