Predatory Plating

Instant - Destroy this: Gain

. Activate this only if you control a card with 6 or more

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Guardwell

You can only activate if you control a card with 6 or more . You control cards in the arena or on the stack.
You cannot activate this ability while in the process of playing a card with 6 or more to pay its resource cost.

Guardwell - When the combat chain closes, if this defended, put -1{d} counters on it equal to its {d}

Guardwell is an ability keyword.
If a card with Guardwell has 0 when Guardwell resolves, no counters are put on the card.
A card cannot have negative . If an effect would reduce a card to less than 0 it only reduces it to 0.
If the card with Guardwell has defended and has 0 when the combat chain closes, no counters are put on the card when the trigger resolves.
Guardwell’s effect resolves before effects that last for “the/this combat chain” end. E.g. Shred’s effect will still continue to apply to the defending card when Guardwell is resolving.