"Stay vigilant, men. I may be running the Maze, but you will all stand united in my absence. When I return victorious, Ravnica will know the triumphs of the Legion!" - Tajic, Boros Maze Runner
This deck is a rounded-out aggro-midrange deck. It's fast enough to succeed against control, it's resilient enough to play against other aggro decks, and it offers enough mid-late game strength against midrange and control that you will always have good options at any point in the game.
Card by card explanation:
Spear of Heliod - +1/+1 for your weenies and pseudo-removal for the cost of 1WW is pretty sweet, especially in conjunction with Assemble the Legion.
Banisher Priest - The best "removal" white weenie has at its disposal. At the very least, she can be cannon fodder and slow the opponent down, or is a 2/2 or 3/3 body.
Boros Reckoner - 3/3 for 3 that can trade fantastically. Super strong defensive option and super strong offensive option.
Daring Skyjek - Not the best card by himself, but with Banishers and other cheap creatures, he can effectively punch through the opponent's defenses early on.
Dryad Militant - A 2/1 for 1 that offers a threatening turn 1 presence. Not as good as our other 1 drop, but she's super relevant now-a-days with the rise of Elixir of Immortality.
Precinct Captain - What more explanation do you need? A 2/2 for WW with first strike that makes tokens. He becomes a huge problem if left unanswered for a few turns, especially if a Spear is on the board. He syncs well with Boros Charm, which can let him double trigger his ability.
Soldier of the Pantheon - White Weenie's turn 1 all star. A 2/1 for W is good for this deck to start, but his ability is relevant on every level. Lifegain against almost every other deck is sweet, and the protection is unreal. This card is super hard to deal with for some decks.
Tajic, Blade of the Legion - 2/2 for 4. Not fantastic. Indestructible 7/7 for 4, $wag. He survives Supreme Verdict, Anger of the Gods, etc. Exile is still viable against him, but this is the case with every other creature. His trigger with Assemble the Legion is bonkers in the late game.
Assemble the Legion - Our window into the late game. If we can't manage to lock down a win in the first few turns, this gears us up for the long haul. At the very least, it gives you tokens to chump with until you can come up with a plan. At its best, it will overwhelm your opponent in 3 turns.
Boros Charm - Charms are outstanding. The indestructible is fantastic in this deck to answer Verdicts, Angers, Gaze, etc. Double strike is sweet with the likes of Tajic, Precincts, and Skyjeks. The burn is great for squeezing out the last bit of damage you need.
Brave the Elements - Do I need to explain this to you?
Magma Jet - Valuable burn that can keep other aggro decks under control and help filter through your draws.
Chandra's Phoenix - Great sideboard option against control. It's not bad against most midrange or aggro decks either, since it can come back again and again after death.
Fiendslayer Paladin - Monoblack hates him! RDW can't stand him! Find out how this lone Knight has been hosing decks for the price of 1WW! But seriously, the lifelink and first strike are great against aggro and his ability is fantastic against Doom Blade, Magma Jet, yada, yada
Imposing Sovereign - Not 100% on having this in the sideboard, but it is certainly relevant against fast decks like RDW or WW. Slowing them down so you can set up for the mid game is crucial, and this lovely lady does just that.
Boros Charm - Against control, primarily, but relevant in other matchups.
Lightning Strike - Good to bring in against decks with lots of creature hate, like Anger, Verdict, Mizzium Mortars. It lets you draw into a damage source that won't die the next turn. Also good against aggro decks with lots of small creatures that you have trouble trading with easily.