Weigraf is arguebly the hardest battle in the game, so a lot of people have had this same same problem. What you need to do is have Ramza a Squire and equiped with the Chameleon Robe. For some unknown reason, Weigraf will not use his abilities (Lighting Stab, ect. I forgot most of them) If you don't have the robe then the battle is just a bit harder. Now, run around the battle feild as far from Weigraf and keep using Accumulate. Once you've gotten your strength high enough, just go in and pummel him with all you've got. Once he's defeated, he will transform into Velius, but don't worry, you other characters will then come in and aid you.