The sideboard of this deck is mostly up to the user's discretion, I myself swap cards around in it back and forth constantly. Most cards in it have copies within the deck, and are for adding what you feel you don't have enough of. However there are a few exceptions:
Apostle's Blessing is a good card for defense against targetted artifact hate, or to get a nice attack through. Side in against creature/artifact hate, or pillowfort styled decks.
Ethersworn Canonist shuts down faster, non artifact based strategies. It slows down multiple strategies that would want to throw a ton at you all at once. It also stops Snapcaster Mage from being as effective.
Radiant Flames is in to hate on token or ridiculous aggro decks, and with the varied mana base, it isn't hard to get two or three damage to everything. Use with caution, it does hit your creatures.
Signal Pest is good to throw in if your opponent is playing a solitaire or non-interactive deck, helping your combat to be that much more damaging to an opponent on turn 2.
Torpor Orb
is a good defense against just about every single Enter The Battlefield trigger based deck. It stops effects like Purphoros, God of the Forge, Snapcaster Mage, Vendilion Clique, Thought-Knot Seer and so many other cards that show up in competitive modern decks.
