Customize one or all of your characters' weapons with the ability "Zombie Strike" and attack Yu Yeven with it. He can be turned into the zombie status. Then with the same or another character use a "Pheonix Down" or cast "Full-Life" on Yu Yevon. This will instantly kill him. It worked for me. His health is about 99,999,999 so have fun with this code and even steal items from him before you kill him.