Auras are enchantments that attach to other cards, usually creatures. In Magic 2015, White and Red have the most auras. Build a deck with up to 21 auras in it, and at least 15 cheap creatures.
You'll want to play against a deck that has trouble killing creatures, so avoid Black or Red decks. Or make a useless custom deck to play against. Put every aura you draw on one creature to unlock the achievement. You don't have to win the duel.