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This is too slow to work, really. If Marowak could hold the Thick Club and Focus Sash/Salac berry at the same time, you'd have a chance. But with 45 base speed, you'll be taking 2 hits before you get a shot off, and you're unlikely to survive both.
Just catch a million Dittos, and of the ones of the correct nature, look for the telltale "alert to sounds" and "likes to thrash about." You'll have to IV test them though, to confirm that the IVs are 31, and not 26, 21, 15, 11, or 6.
So that it survives long enough to make the transformation. If you think about it, thats why the Ditto items boost defences and speed, rather than attack.
Defensive Pokemon are defined this generation more by their resistances than by base stats. Bastiodon's defense, for example, looks far less impressive when it takes 4x effective hits from Fighting or Ground Pokemon. When you consider how many big physical threats carry Earthquake or Close...