Is adjusting damage type an appropriate power level for bestow curse?


A rather creative cleric in my game’s party has presented with a new curse they want to use. The new curse is:

All damage the target creature takes is treated as the damage type to which the creature is most vulnerable

This curse basically removes any sort of damage reduction from the game, plus it can even make all damage the players do to monsters more powerful than intended. Our evoker, for instance, can memorize only powerful fire spells with confidence that those fire spells will have full effectiveness on creatures. The cleric is prepared to heighten spell to keep the saving throw as high as possible.

Is this curse:

  1. Appropriate for bestow curse
  2. Too strong for bestow curse but appropriate for bestow curse, greater
  3. Inappropriate for a permanent debuff

I don’t consider this a homebrew question since bestow curse explicitly encourages creation of new curses. I have just always had difficulty judging the power of curses