Slain: Back From Hell

Shortly after you reach The Hive, you'll run into three flying monsters that shoot green projectiles at you. Those are the Beholders. Dispatch the first three you come to without magic (RB). Next, you'll find yourself locked in a room where 5-6 Beholders will spawn above you. Evade and attack, striking back whatever green projectiles you can to help make quick work of them. So long as you don't press RB and cast any magic, the achievement will pop when the last one dies.