This is where the real fun of playing this deck is!
There are so many cards you can cast off Sunforger, and therefore there is great flexibility in building the sideboard as well. You can go for modern staple white and red instant cards that are easy to hardcast and therefore friendly to board in (which is probably the most competitive approach), you can go for a host of powerful controlling cards and counterspells or you can go for weird complex tricks. Here is my current sideboard:
Advent of the Wurm: Instant body, just nice to have. I find it's never a bad idea to throw in some additional access to bodies in decks that are thin on creatures like this one.
Boil: Make blue decks hate you.
Crackling Doom: A good way to deal with creatures that can't be removed with spot removal. Typically indestructible or hexproof creatures etc. The great thing about this spell, compared to alternatives like Blessed Alliance or Dispense Justice, is that it forces your opponent to sac their largest creature and also can be used before attacks to avoid attack triggers from eldrazi titans and such. This makes it much better at dealing with Emrakul or bogles. Another great thing is that it is "each opponent" so it doesn't need to target the enemy player, which makes it work through things like Leyline of Sanctity.
Hide / Seek: A very flexible little card, get rid of that mindslaver in a way that can't be brought back with academy ruins, remove Emrakul from their deck in response to Nahiri/Tooth and nail/Polymorph etc. And against Angel's Grace and such you can finish the infinite combo in style by just chaining seek to remove their whole deck and pass the turn.
Jund Charm: Graveyard hate and wipe against wide boards. One great thing about it is that if you can cast it twice in a turn (with wheel) you can use the first casting to protect your enabler with the +2 counters so it survives the wipe.
Kolaghan's Command: Just plain good modern staple solving many kinds of problems.
Ojutai's Command: Very versatile answer, great for bringing back Paladin and for stabilising against aggro.
Punish Ignorance: Probably the most flavorful card to pair with research, and it's also a pretty good counterspell when cast through Sunforger. A little clunky to cast from the hand sometimes so it should be used mostly when you can chain reseach into a sunforger activation the same turn.
Render Silent: Great counterspell, if you get this going with wheel/forger it resembles an isochron/silence lock as well. This is also the best counterspell against Bloodbraid Elf and other cascade cards as the silence effect stops the cascaded spell from being cast.
Scour: Supercharged enchantment removal, one of the more common cards to board in game two expecting Stony Silence.
Settle the Wreckage: So powerful with Sunforger it's not even funny.
Utter End: Sometimes you just need to remove something without compromises, planeswalkers usually.
Wear / Tear: Since we really don't like facing cards like Stony Silence, Pithing Needle and Damping Matrix, it's good to have some hate that is also easy to cast from our hand, so this goes in game 2 against white decks pretty much every time along with hide/seek.
Wrecking Ball: Land hate, can also kill creatures.
