I put him as ninja and gave him guts. Every turn I used the one that gives you speed+1 (cheer I think but maybe not) and ran away so that Wiegraf couldn't hit me. If you keep boosting speed you'll end up with two turns in a row and that's when you go for the kill. Just make sure when you are boosting you stay far enough away that his sword attacks won't get you.

This is always the hardest battle of the game for me by far.