Gorgonic Guardian

Regarding the effect: "Once per turn, you can detach 1 Xyz Material from this card, then target 1 face-up monster your opponent controls; change its ATK to 0, and negate its effects until the end of this turn. This is a Quick Effect."

This is a Quick Effect that can be activated while this card is in a Monster Zone.

Detaching 1 Xyz Material from this card is the cost to activate this effect.

This effect targets 1 face-up monster in your opponent's Monster Zone.

This effect can be activated during the Damage Step, from the start of the Damage Step up until before damage calculation.

You can target a monster that is not an Effect Monster, or a monster whose effects are already negated, as long as its ATK is not 0. (The targeted monster's ATK becomes 0 when the effect resolves.)

You cannot target a monster with 0 ATK.

Regarding the effect "Once per turn, you can target 1 monster on the field with 0 ATK; destroy that target": * This is an Ignition Effect that can be activated in the Monster Zone. * This effect targets 1 face-up monster with 0 ATK in the Monster Zone. * If the targeted monster is in face-down Defense Position or its ATK is no longer 0 when the effect resolves, the effect is still applied normally and the targeted monster is destroyed.