First off, I would like you to know that I am nuts about Boros! I love the guild leader, I love the battalion mechanic, I love everything about Boros! Next, Firemane Avenger was the first ever Gatecrash Boros card I saw. At first I just looked at it, but then I realized that it was totally awesome. For four mana you get a 3/3 flying and battalion that lets you deal three damage and gain three life. When I saw Spark Trooper, I thought it would be the second rare for the Intro Deck. I looks sort-of like a junk rare, and Wizards doesn't normally put really good rares in Intro Decks. I was surprised. Foundry Champion and Assemble the Legion work great together. Get an army of 1/1 soldiers then-bam-deal a bunch of damage with Foundry Champion. Also, he can become a huge attacker of a great defender.
Finally, you can see the great Boros Battalion.