You should do it on your turn or during his upkeep, otherwise he can cast it in his main phase since he has priority and by the time your Purify the Grave can resolve, the card is no longer in the graveyard for you to target. Once he casts something from the graveyard with Flashback, it comes out of the graveyard and goes on the stack.
He taps 4RR to Flashback Rolling Temblor. Do I have an opportunity to cast Purify the Grave targeting Rolling Temblor before Rolling Temblor's Flashback can occur, or is there no gap between tapping and using Flashback where I can respond?
Tapping a land for mana does not use the stack and cannot be responded to nor can the payment of costs.
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Trade Thread
Standard
UB UB Control UB
UW UW Pristine Control UW
UW Venser Control UW