Crota is only able to be killed by a sword because of the Sword Logic. Sword Logic is an art in which you gain power from your killings, and while it can be gained in the material world (if practiced), it only comes into play inside Hive Throne Worlds (special pocket dimensions created by incredibly powerful Hive entities). Inside of Throne Worlds, the Sword Logic is a physical law that affects all residents, even Guardians. When we enter Crota's Throne World, we have little to no power since we do not practice Sword Logic in the material world. We do gain some from killing ascendant and hallowed Hive, such as Swordbeares and Gatekeepers, but even that power is nothing compared to the power Crota wields. In fact, we only have just enough power to bring Crota down to his knees with our combined firepower, but we are unable to do anything to actually hurt him. However, ascendant Hive blades found in Hive netherworlds can gain their own power via the Sword Logic. [spoiler]Eris, Eris, what a name, a name for discord, a name for far cold orbits where no living thing should dare to go. I like this name. Let me give you a gift, Eris. Let me tell you about the power in the logic of the sword: A Shredder or a Boomer is a powerful weapon, but it kills acyclically. You see? It sends out harm and it takes nothing back. The bolt passes away into nothing. A sword, though, a sword is like a bridge, a crossing-point. The sword binds wielder to victim. It binds life to death. And when the binding is done—the sword remembers. When the Boomer's fire has burnt away into axion and neutrino scatter, the sword goes on, hungrier and sharper. Understand that this nightmare logic underpins His nightmare world, and you will see why the ascendant blade has so much power there. Whenever in our passage we find ourselves in need of power—remember that the greatest authority here is a blade made keen by eons of use. (Ascendant Sword)[/spoiler] When we are armed with an Ascendant Sword, we are powerful enough to hurt Crota, and it is with the sword that we kill him.