Doing as much as possible in Boros.
Edit 5/5/15:
-Angelic Field Marshal, Figure of Destiny, Conquering Manticore, Molten Primordial, Darksteel Pendant, Three Dreams, Curse of the Nightly Hunt, Sunbond, Unquestioned Authority, Plains
+Hoarding Dragon, Phyrexian Devourer, Stonecloaker, Sunscorch Regent, Treasonous Ogre, Sigil of the New Dawn, Mistveil Plains, Sarkhan's Triumph, Tithe, Chain Reaction.
I ended up taking out a number of the underperforming cards and cards that weren't synergistic with the deck's main goal. In their place, better fling targets in Phyrexian Devourer and Sunscorch Regent, better combo enablers in Treasonous Ogre and Sigil of the New Dawn, tutors in Sarkhan's Triumph and Hoarding Dragon, and more utility in Stonecloaker, Chain Reaction, and the Tithe/Mistveil Plains combo for sunforger.
Recursion:
Feldon of the Third Path, Adarkar Valkyrie, Sun Titan, Mimic Vat, Nim Deathmantle, Gift of Immortality, Sigil of the New Dawn.
These help repeatedly recur the creatures flung by Brion.
Fling Targets:
Phyrexian Devourer, Hamletback Goliath, Malignus, Sunscorch Regent, Moltensteel Dragon, Scourge of Geier Reach, Serra Avatar, Solemn Simulacrum, Yosei, the Morning Star, Zealous Conscripts.
Each of these creatures offers either unusually large bulk, or a powerful enter the battlefield/leave the battlefield ability, and a recursion chain onto any of them can often win the game.
Ramp:
Burnished Hart, Knight of the White Orchid, Solemn Simulacrum, Weathered Wayfarer, Boros Signet, Fellwar Stone, Sol Ring, Endless Horizons, Tithe.
Keeping up with other decks in Boros can be difficult, so the more ramp, the better. As important as ramp is, so also is making sure to hit each land drop, hence the inclusion of cards like weathered wayfarer and endless horizons.
Draw/Filter:
Humble Defector, Orcish Librarian, Avarice Amulet, Crystal Ball, Mind's Eye, Staff of Nin, Well of Lost Dreams.
Boros often suffers from lack of card draw, so Artifacts are a solid answer. Scrying unnecessary cards away can also act as a form of draw, improving the overall quality of the cards you draw.
Steal Effects:
Preacher, Zealous Conscripts, Harness by Force, Mark of Mutiny, Threaten.
Stealing individual creatures allows for Brion to sacrifice them for fun and profit. Especially tasty are steal effects attached to creatures, which can be recurred to continually steal your opponents' best creatures.
"Win the Game'' Effects:
Serra Avatar: Fling Serra avatar, deal damage to target player equal to your life total and double your life total. The easiest combo in the deck.
Phyrexian Devourer: Repeatedly use Phyrexian Devourer's activated ability to make it arbitrarily large, then fling it. Its triggered ability is irrelevant since you can fling it with Brion before it resolves.
Moltensteel Dragon: Pump Moltensteel Dragon up, swing in with it, then fling it with Brion afterwards to get double value out of the phyrexian-breathing effect, and get some of the life you spent back.
Yosei, the Morning Star: Lock out an opponent with a continually recurred Yosei, tapping all of their permanents and preventing their lands from untapping.
Insurrection/Mob Rule: Simply wins the game on a clogged board.
Phyrexian Rebirth: Wipe a board lategame, then recast Brion and fling the giant token left over.
Grafted Exoskeleton: Either give Brion another 4 power and swing in for lethal, or toss any 10-power creature (hint hint, Moltensteel Dragon), to deal lethal infect damage. Tutorable via Stonehewer Giant.
Phyrexian Processor: Get to 9 mana, cast the Processor, spend all but one life, poop out a token, and fling it with Brion, regaining all of the spent life, and dealing damage equal to your life total minus one to a target player. Obviously, leave a bigger buffer if you're worried about an opponent pinging you down while you're vulnerable. Each subsequent turn, you'll get another fling for your life total for only 5 mana.
Protection:
Grand Abolisher, Sunforger plus attached instants, Darksteel Plate, Swiftfoot Boots, Whispersilk Cloak, Stonehewer Giant to tutor for any of the aforementioned equipment, Legion's Initiative. Legion's Initiative even dodges boardwipes, both yours and opponent's.
