Look up, immediately. A Drone swarm will descend upon you from holes in the roof, followed by a pair of Elites from the right. Try and get into the top right door and use your sword to cut the Elites down as they spawn. Note that one of the Elites is a Major, who you will have to lunge and finish with a headshot. Once they're dead, another pair of Elites will spawn in the hall; give them the same sword-and-headshot technique. Often, a Major Elite will run into the hall from the outside, but he should fall just as easily.