A Dragoon with Squire abilites works well. Keep your distance and use Yell to speed Ramza up, once Ramza starts getting two or three turns to Wiegraf's one, let loose with the jump skills. Make sure you have Move+1 or Move+2 with Gremnias Boots to make keeping away better.

Belias is only hard if you decide to focus on the Ultima Demons first. If you just go straight to Belias, you should take him down pretty quickly.