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Deals damage and has a 30% chance to flinch the target.
Astonish is such an astonishingly weak move that nobody in their right mind would ever dream of using it. Even though physical Ghost-type moves are hardly strong, you are better off with Shadow Sneak or Shadow Claw, which themselves can hardly be recommended. As far as flinching goes, you are best off with the likes of Iron Head, Air Slash, and Rock Slide.
Shadow Punch is a more powerful move, which will always hit the target.
Shadow Claw is a more powerful move with a high critical hit ratio.