Here is the way i kill wiegraf/velius : Have ramza in the class you wish him to master (or level a lot) then equip every item that boost speed and movement range.
His second skill should be guts (yell and acumulate are a must), give him move +1 or higher.
In the fight, use yell at every turn but stay at a square of two (or more) from him and his lightning stab. After a while using Yell you should be able to act 7 time before he act once. at that point use acumulate to have 50 to 99 power (99 give lots of sp and xp). then, then wait his turn. After his move, get behind him and strike (1-2 hit kill), he will morph but you still have 5 turn left before anyone move. get back him and strike (again 1-2 hit kill) and Ta-dam! you have killed Velius and mastered (or near) the class of Ramza.