Call it instinct if you wish, or common sense when a lightsaber is being swung at you, but your lightsaber connects with his in a solid block. The blow is strong... and very real! Your alter ego continues to assault you like this, employing forceful attacks to drive you back and weaken your defense.
You cannot defeat me! Not without the Dark Side of the Force!
The lightsabers crash back and forth, but he has an edge on you. His dark power is tiring you faster, your arms being to feel sore from the blows that cause your arms to shudder. You cannot keep him from driving you back, pushing you into the darkness.
A hopeless quest for a Jedi to take up! I am more powerful than any Jedi...
He is laughing, a crazed look in his glowing golden eyes.
The duplicate is pushed back, allowing his lightsaber to lock with yours. He starts to twist the lightsabers back towards you, pushing for your throat...
Come, now, Xavier. You know EXACTLY what it will take to defeat me. And to defeat Friste! You cannot fight this any longer. Give in to your anger, and strike me down...
Xavier slackens on his powerful guard and lets the duplicate stumble towards him, before tripping him up and letting him land on the ground... but the spectre melts and appears behind you. You just narrowly manage to block the incoming lightsaber.
If you will not give in, you will die. That is your fate if you reject the Dark Side.
Instead of rejecting your anger as Master Cabas taught you so many years ago, you accept it. You feel yourself become stronger and the Force rejuvenate you after this tiring, one-sided fight with your doppelganger. When you force away his lightsaber and swing back, it is he that has fallen to the defensive and given ground.
Excellent! You are achieving your potential, Xavier. Let your emotions fill you with strength, not weakness! Kill me! KILL ME!
And you finally do. You rain powerful blows down upon your foe, more powerful than you remember being capable of... until finally your lightsaber cleaves straight through his skull and down the center of his body. He collapses to the ground, and the darkness fades as you stare at the melting figure staring vacantly at you from his split face...
"Xavier," Friste says. "I sense you have passed the test..."