could use a trigger to use a dummy caster to use a cripple-based ability to target the enemy unit
if it wasn't a hero ability, you could just implement moon glaive, the perfect dummy passive ability (that ability never does anything lol)
if hero, give cripple multiple levels if the defense drop is the altering factor, alter the level of the ability when you create the dummy caster (maybe even only give the ability to the dummy caster via trigger as well, i found that to be handy for only using one dummy unit ever) and order it to cast it on your target when they take damage
i like item abilities like that, but they're too buggy unfortunately :V