Yes. The normal procedure of casting a spell is not changed. So your opponent chooses the value of X=15 and later exiles a green card with CMC 15. For an X-spell on the stack, the CMC is calculated using that value. So Shoal has a CMC of 17 in this case, which is 4 or greater, so Disdainful Stroke can target and counter it.
Note, that the exiled card does not determine the value of X, rather it is the value of X that determines what can be exiled to pay for the spell.
GWUBRDraft my Old Border Nostalgia Cube! and/or The Little Pauper Cube That Could!RBUWG
Modern:WDeath & TaxesW | RUGRUG DelverRUG
Note, that the exiled card does not determine the value of X, rather it is the value of X that determines what can be exiled to pay for the spell.
Former Rules Advisor
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