To defeat Wiegraff at Riovanes Castle, become a monk and learn WAVE FIST, CHAKRA and MOVE-HP UP. A Bracer will help as well.

Start out by getting as close to him as you can. Then use wave fist. He will then use Lightning Stab and run away. DON'T FOLLOW HIM. Run back to where you started and use chakra. He will then move to where he started. Now repeat all this until he's dead.