So after you clear out the two rooms at the beginning you pick up the chalice, then your team focuses all the fire you have on the swordbeaer. If you do this just right his sword will drop, if this happens someone must sprint to pick it up while the rest of the team shoots crota. For some reason your bullets take his shield down and you have this super short glitchy window of time to do damage on him with the sword. Do this a couple times and he dies. Hopefully bungie patches this, if you didn't disconnect then you didn't do it legit.