When you reach the part with a Hornet, proceed and clear out all of the Brutes before going inside the tower. After the Elites land, go up to one of them and bash them, but not in the back or they will die. Bashing them will take their shield down. After that, throw a plasma grenade at the Elite and make sure it sticks. Since he is about to die, he will take out his energy sword. When he dies from the grenade, take his sword. Note: This will not work with any other grenade. This procedure will also work with the Arbiter. When you get the energy sword it will only have about 55 energy in it. If you obtained one from the Arbiter it will only have 2 or 3 energy in it.