Starfield Veil

Instant - Destroy this: The next aura you play this turn enters the arena with a holo counter. Activate this only if an attack has fragmented this turn.

Guardwell

Rulings

From the Omens of the Third Age release notes

You can only activate Starfield Veil’s activated ability only if an attack has fragmented this turn (an attack’s fragment triggered effect has resolved). This includes your attacks and your opponents’ attacks.
When Starfield Veil’s activated ability resolves, it will generate an identity-replacement effect for the next object with the aura subtype you play this turn. When that aura resolves and enters the arena, that aura will enter the arena with a holo counter.

Keyword notes

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.