I am starting with 2 black/red creatures. I add all black creatures minus the two using scavenge, and all black non-creature spells.
That's a total of 8 creatures, 2 sorceries, 2 instants, 3 enchantment auras, 1 enchantment and 1 keyrune: 17 cards. Now I just need 6 red cards.
I can add four red cards without reservations. Pursuit of Flight is a light card but has a blue ability and is therefore more suited for an Izzet deck. I need two more cards.
I choose the one Pyroconvergence and one of the two Batterhorn. The former is better because for the latter I may have opponents that do not have any artifacts in their decks.