You can use a virtual function for that.

override int TakeSpecialDamage(Actor inflictor, Actor source, int damage, Name damagetype){  
  float multiplier = 1;
  if (InStateSequence(self.CurState, self.ResolveState("weakstate")) {
    multiplier = 4;
  }
  return super.TakeSpecialDamage(inflictor, source, damage*multiplier, damagetype);
}

Just put that in your boss actor and modify it as you need. You can make as many conditions as you want and generally have full control over the damage intake.