i find that the best way to defeat them is 2 make sure that Whisper is attacking one, so that it's attetion is on her, and shoot at the other one with your bow, i find that using the slow time spell works really well, cause you can see the boulders coming at you and you can get off almost twice as many shots as if you didn't use it.
John-117: "Sir. Permission to leave the station."
Lord Hood: "For what purpose Master Chief?"
John-117: "To give the Covenant back their bomb."
- Cairo Station final cutscene.