1) If I exiled Walking Ballista using Urza's activated ability can I play it normally or do I have to default to casting it "without paying its mana costs" ie. making it a 0/0
2)If I do cast Walking Ballista for 0 and hold priority, can I dump my infinite mana into Walking Ballista's ability to counters? will it survive and allow me to remove those counters for infinite damage?
Just a curious interaction that I want more clarification on. Any help will do.
1. You play the card "without paying its mana cost". Note, however, that in general, you can play that card this way only any time you could normally play a card of its type (C.R. 117.1a for spells; C.R. 116.2a, 305.2 for lands). (For further information, see this thread; see also this thread.) Note also that, in general, if you cast a card with X in its mana cost (such as Walking Ballista) due to Urza's last ability, the X will be 0; that ability won't let X be anything other than 0 (C.R. 702.84a, 107.3b). (Currently, the only exception is Spoils of War [C.R. 107.3c]. See also this thread.)
This means that in general, in the case of Walking Ballista—
you can cast Walking Ballista with Urza only any time you could normally cast a noninstant spell, since Walking Ballista is neither an instant card nor a land card (C.R. 117.1a),
as you cast it this way, you can't choose any value of X other than 0, and
if you cast it this way, then upon resolving, Walking Ballista will enter the battlefield without any +1/+1 counters on it.
(Note that, in general, a 0/0 creature will go the graveyard as a state-based action unless something is raising its toughness [C.R. 704.5f].)
2. In general, no. If Walking Ballista enters the battlefield and X is 0, it will generally enter the battlefield without any +1/+1 counters on it, so that if nothing else is affecting Walking Ballista's toughness, it will go to the graveyard as a state-based action (for having a toughness of 0 or less) before any player gets priority to activate abilities, including Walking Ballista's second or third ability (C.R. 117.5, 704.5f; review C.R. 117.5a).
1) If I exiled Walking Ballista using Urza's activated ability can I play it normally or do I have to default to casting it "without paying its mana costs" ie. making it a 0/0
2)If I do cast Walking Ballista for 0 and hold priority, can I dump my infinite mana into Walking Ballista's ability to counters? will it survive and allow me to remove those counters for infinite damage?
Just a curious interaction that I want more clarification on. Any help will do.
This means that in general, in the case of Walking Ballista—
(Note that, in general, a 0/0 creature will go the graveyard as a state-based action unless something is raising its toughness [C.R. 704.5f].)
2. In general, no. If Walking Ballista enters the battlefield and X is 0, it will generally enter the battlefield without any +1/+1 counters on it, so that if nothing else is affecting Walking Ballista's toughness, it will go to the graveyard as a state-based action (for having a toughness of 0 or less) before any player gets priority to activate abilities, including Walking Ballista's second or third ability (C.R. 117.5, 704.5f; review C.R. 117.5a).
EDIT (Jan. 31, 2022): Correctness edit.