There is a cheese. Search for a video from Esotterick called Calus Cheese or something similar and choose the under the stairs method. It's the only way that won't drive you crazy. They expect you to grapple around the arena constantly getting a peck at Calus in here and there for an hour to whittle him down. Bungie sucks. Destiny has from inception always been a medium difficulty campaign followed by endgame that has increased difficulty. The new "leadership" is a gatekeeping tool.