That would defeat the purpose though. First, because they could just heal themselves, second because they would die faster if not at full life, and third, doing direct damage makes it very easy for the party to finish him off before 3 turns are up. It is much better done through a script, and as NAMKCOR is making one, that seems fine.
As well, you would actually need a script to modify the Slip Damage, since in XP the damage done is set. There is no non-scripting way to directly modify the slip damage in states