``I want to bring `chaos' into the world... to hear the screams of humans... But, don't worry, Ramza. I'll kill you... myself!'' -- Wiegraf
The battle with Wiegraf/Velius in Riovanes Castle is one of the hardest battles in Final Fantasy Tactics, because Ramza must fight Wiegraf alone. One strategy for completing this battle is to set Ramza's job class to Squire, and have the Yell and Accumulate squire skills, as well as a mechanism to heal (you'll need it for the repeated Earth Slash attacks). Auto Potion (chemist) or Regenerator (priest) work well here. As long as Ramza can survive Wiegraf's initial attack (usually a Lightning Stab), this battle can be completed.
The trick is to exploit a bug in the AI that permits a cycle; Ramza's first move should be to the tile directly SOUTH the steps to the southeast (where north is the top of the room). Wiegraf will move to a position northeast of the torch, into the water; Ramza's next move should be one tile WEST; Wiegraf will move in lock-step, attacking with Earth Slash each time. In this manner, Ramza can cycle back and forth as much as necessary to Yell up to SP 50 and Accumulate up to AT 40 or so; from that point on, the battle is trivial, since at AT 40 and 6.5 turns to his one, you can kill Wiegraf in one attack and kill Velius in at most 3...
Wiegraf will attack during this cycle with Earth Slash, so make sure to heal periodically unless you have Auto Potion or Regenerator... it is possible to complete this battle when Ramza is under level 30...