The technique I used is actually a very simple one. Have a samurai equipped with Move HP-Up and HP Restore. After he attacks you, move close to him to where you're only one step away from him and use Heaven's Cloud. After taking his last attack your samurai should have enough HP to take at least one more hit. Weigraf should attack you and then move away from you since you're so close to him. (He's quite a chicken.) After Weigraf attacks you again your samurai should use HP Restore and regain all your HP. If it doesn't work then just start over until it does work. For your next turn move close to him and use Heaven's Cloud again. After Weigraf attacks you move as far away from him as you can and use Murasame to heal yourself. Now Weigraf shouldn't be able to reach you to perform his attack so his turn will be kind of a waste. Now move close to him and perform Heaven's Cloud one more time and he should be defeated. This is what I did.
