I'm stuck in the Ikana Castle at the two Guards and boss. How do you beat..

First, shoot fire arrows at the curtains on the windows to burn them off, letting the sunlight in, then begin fighting with the two Soldiers.

Hit them with your sword and when they fall to the ground, reflect sunlight onto them with your Mirror Shield. Once they are both gone, the King will get up and fight you himself. He is a lot tougher than his Soldiers are, but he only has three basic attacks: his main attack is his leap attack with his sword, it isn't as dangerous as the other two, but it can do so heavy damage.

He also has an attack where he decapitates himself and sends his head to attack you while his body attacks from the ground. His final attack is blowing his poisonous breath on you, which does quite a bit of damage to your health. The easiest way to defeat him is to just attack him with your sword until his guard is down, then stun him with the sunlight.

You will then see a clip of the two Soldier Ghosts arguing, then they disappear, followed by the King's Ghost who teaches you the Elegy of Emptiness which allows you to create one stationary clone of yourself per form. Hope that helps!