Or...just not use it, because its a terrible spellI think Morrus answered the question, but I want to point out HP spells are still effective on creatures with higher HP. You just have to wait until you drop the creature below that Threshold. This is the case with O5e as well. I mean Power Word Kill (a 9th lvl spell) requires the opponent to be 100HP or below, yet the monsters you would face at that level will have hundreds of HP. You are expected to whittle them down a bit before you use it.
