The only thing you can´t make in the database is that you can´t make him weak against light and you can´t make him get damage instead of healing. Somewhere in your Game_Battler script (don´t know which number) in "def skill_effect" should be these lines:
last_hp = self.hp
self.hp -= self.damage
effective |= self.hp != last_hp
effective |= states_plus(skill.plus_state_set)
effective |= states_minus(skill.minus_state_set)
change it to
last_hp = self.hp
self.damage = -self.damage if self.damage < 0 and self.state?(ZOMBIE_STATUS_ID)
self.damage = (self.damage * 1.5).to_i if skill.element_set.include?(LIGHT_ELEMENT_ID) and self.state?(ZOMBIE_STATUS_ID)
self.hp -= self.damage
effective |= self.hp != last_hp
effective |= states_plus(skill.plus_state_set)
effective |= states_minus(skill.minus_state_set)
Replace ZOMBIE_STATUS_ID and LIGHT_ELEMENT_ID wth the appropriate IDs from your database.