Choice Scarf is the most desirable item, as it allows you to hit first the foe you Transform into.
Nature and all EVs except for HP will not matter much if you are Transformed. However, since the foe might prevent your Transformation, you'll want to switch out, or if that is not possible take down the Sub (or damage Zoroark) as soon as possible. The only way Ditto can do this is through Struggle. As such, I would suggest giving it an Adamant nature and 252 Attack EVs and 4 Speed EVs (in addition to 252 HP EVs). Also, Ditto does not copy the Hidden Power type (as it does not copy IVs), so I would suggest running an HP Ice.