It is possible to obtain the Moonlight Sword without killing Guyra. First, in preparation, fill all of your crystal flasks, have at least one key in a guidepost and a gate to match it. Then, make your way to Guyras chamber. When you first get there walk straight towards and strife to the left making sure he doesnt knock you off. Now when you are as far left and as far forward wait until Guyra is completely facing you. Heres the tricky part. Quickly run towards Guyras other side and get on the same platform as him, now make a short jump across to the platform where the Moonlight sword is. Now quickly grab the sword and use the gate to get out before Guyra has time to turn around and knock you off the platform. If you did this successfully you should proudly be able to use the Moonlight sword.