You can target one face-up monster on the field once per turn that loses 500 defense and attack, with your opponent unable to activate cards or effects in response to the activation of this effect
Playing a Fireblast in response to a Natural Balance didnt just offset the sacrifice two Mountains cost, it actually netted two mana, since the lands from Natural Balance entered the battlefield untapped

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Potentially, this might be exactly whats happening here, or Gwenom may simply be overpriced