‘Onslaught’ Review: Adria Arjona and Dan Stevens Star in Adam Wingard’s Fun but Scattered Military Slasher

A 12-hour clock counts down to an inevitable massacre in “Onslaught,” the kind of elevated, razor-wire genre romp that director Adam Wingard and writer Simon Barrett used to make before studio outings like “Godzilla vs. Kong.” It isn’t the…