W Heroic (Pauper)

Lands: I tried playing 17 lands (16 Plains + 1 Steppe), but it didn't feel solid enough. 16+2 is fine, maybe 17+1, but I'd always keep at least one cycling.

Creatures: Trailblazer, Seeker and Skyguard must always be there.
I'm not really sure on Deftblade Elite, I also tried putting Standard Bearers in the mainboard and they both have their pros and cons.
Wingsteed Rider (2/2 flying with Heroic for +1/+1 counters) is worth trying, but CMC 3 is already too high for this deck.

Enchantments:
Ethereal Armor is a must-have Aura, I would never put it out.
Hyena Umbra is also great because of Totem Armor. Depending on the matchup is can be sided out, but I would not play without at least 2 copies.
Cartouche is good, particularly against black or tron decks playing Ulamog's Crusher. Can be sided out against other matchups.
Lifelink is a big maybe. I personally like it because the deck doesn't always start as fast as intended, so this gives you some extra time.
Gift of Granite is another maybe. Becoming a +1/+3 on creatures with Heroic/Prowess is a fair protection against bolts, but nothing against removals. Cho-Manno's Blessing is better, but I preferred the low cost here.

Instants:
Emerge Unscathed is worth +2/+2 on Heroic creatures, plus full protection from a color. Really good.
Mutagenic Growth is a +3/+3 until end of turn for 2 life, absolutely great.
Defiant Strike is good because of the draw effect.
Double Cleave is an excellent finisher.

Sideboard:
As said, Cho-Manno's Blessing is a great protection. Plus, it can be played on enemy creatures to make their auras/buffs completely useless.
Circle of Protection: Red is a response to burn decks. Pretty bad matchups anyway, we're fast but they're faster.
Dust to Dust is the ultimate hate against Affinity and some other decks.
Fragmentize finds place against auras, affinity, tron, mirror...
Journey to Nowhere is there for big creature threats.
Sacred Cat is a nice inclusion against a variety of decks, such as creature-based aggros and those including sac effects.
Shield of Duty and Reason is only useful against green and blue decks.
Standard Bearer against bolts, removals or aura decks.