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A transforming double-faced card enters with its front face up by default, unless a spell or ability instructs you to put it onto the battlefield transformed or allows you to cast it transformed, in which case it enters with its back face up
Because you're transforming an existing creature into Archfiend of the Dross, the opponent's Archfiend doesn't enter the battlefield, so it doesn't get any oil counters
This one is a little tougher to cheat, as you'll have to spend mana if you want to trigger its 'enters the battlefield' board wipe
Hexproof creatures are still vulnerable to board wipes, but given that Snakeskin Veil can be played as an instant , you can use it to shut down a targeted spell first, and hope that your opponent doesn't have the mana or cards in hand to follow it up with anything else