石殴りの巨人 {3}{R}{R}

クリーチャー — 巨人

{6}{R}{R}：怪物化３を行う。（このクリーチャーが怪物的でないなら、これの上に＋１/＋１カウンターを３個置く。これは怪物的になる。）石殴りの巨人が怪物的になったとき、このターン、対戦相手がコントロールする飛行を持たないクリーチャーではブロックできない。

5/4

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Only the English version of a Magic card receives Oracle updates and errata. View this card in English. (Scryfall note)
If blockers have already been declared when Stoneshock Giant's last ability resolves, those blocks won't change or become undone. (2013-09-15)
Your opponents can't assign any creature without flying to block that turn, even if that creature had flying or wasn't on the battlefield under one of your opponent's control when Stoneshock Giant's ability resolved. (2013-09-15)
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens. (2013-09-15)

Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous. (2013-09-15)
An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves. (2013-09-15)