Diablo III has a set of three Achievements that can be earned by killing a certain number of Champion monsters. The three Achievements in this set are called “The Takedown”, “Not So Tough Now”, and “Champion’s Collection”.
Champion monsters are blue (and typically come in packs of three or more). If you play Diablo III long enough, you will eventually earn all three of the Achievements in this set. Just keep going after the champion packs!