Magmatic Carapace

Whenever you play an aura, you may

this and pay

. If you do, create a Seismic Surge token.

Guardwell

Magmatic Carapace’s first ability triggers whenever you play an aura. This is an optional effect: you choose to tap it and pay when the triggered effect resolves. If you tap it and pay a resource this way, you will create a Seismic Surge token.
Because it is an optional effect, you must be able to both tap Magmatic Carapace and pay . If you cannot do both, you cannot choose to do the optional effect.
When resolving Magmatic Carapace’s triggered effect, there is no priority between any of the effects generated (e.g. tapping it, paying a resource, creating a token).

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.