When the Blades ask you to kill Parthanax, approach him to check if he is on the stone wall perfectly. Attack him with any weapon (destruction magic also works) you wish to increase. If he dies, restart and try again. If done correctly, he will just take the hits then fly up and back down. He never dies and you will not get any bounties for attacking him. Use the following trick to increase a one-handed weapon skill. When you are in Helgen Keep, have a Sword Of Imperial and Ralofs asks you to open the gate, just hit him with any one-handed weapon.