Testament of Valahai

If you control three or more Seismic Surge tokens, this gets +2

. If you control six or more, instead this gets +4

.

Guardwell

Testament of Valahai’s ability generates a conditional continuous effect that is always checking as long as it is face-up in the arena. If you control three or more Seismic Surge tokens, this gets +2. However, if you control six or more Seismic Surge tokens, this will get +4 instead of +2
If you control 3 Seismic Surge tokens when this defends, then you lose one of those tokens, this will lose the +2. Similarly, if you control 6 Seismic Surge tokens when this defends, then you lose one of those tokens, this will lose the +4.
If a Seismic Surge token is created during the damage step and causes the if-conditional effect(s) to become active (e.g. 2 to 3; 5 to 6), it will get the +2/+4 but the damage will not be recalculated.

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.

Defense1