I don't know if this will work for you or not, but this is what I did:
Make sure Ramza is about Lv 35 or more, and make him a Ninja (if you haven't gotten that, yet, try to do so, and hopefully you didn't save directly before the fight with Wiegraf so that can be taken care of), and then use Agrias (hopefully with Holy Explosion, or Hallowed Bolt at the least), a Summoner (with a possible Bahamut, or Odin at least, as well as Moogle or Fairy), another Ninja, and a high level Geomancer or Dragoon. In the first one on one between Ramza and Wiegraf, when you're out of range of attacks, throw the best shuriken items you have, and make sure he can use Hi-Potions when necessary. When you can get close enough, nail Wiegraf a couple times from the side, so it's more difficult for him to block. Within about 3 turns, he should go into his Lucavi form, Belias/Velius. At that point, even though the demons show up, don't focus so much on them. Focus your attention on Wiegraf, using the most powerful attacks you can. Attack him physically with Ninja Ramza and the other Ninja, use Agrias' best Holy Sword abilities, summon Bahamut, and attack physically/jump with the Geomancer/Dragoon, depending on which you have. Wiegraf should go down in about 5-10 minutes, if that. Hope this helps.