This is is the hardest fight! I reccommend you'd be at level 40 or so. What I use is Ramza being a monk, I have two ninjas, one with Items as secondary skills, the other Draw Out. I also have a priest and summoner. First off, use the Draw Out Kiyomori for an instant Safe/Shell protection, you'll need it to survive Velius' Cyclops, which usually does 250 - 300 + damage to anyone in range. The Archaic Demons will attack using Giga Flare, Dark Holy and Life Break (Very dangerous!). Ignore them no matter how much they hurt, concentrate on Velius. Your priest and summoner should use Holy and Lich respectively. If everyone has Auto Potion as Reaction skill, then don't bother wasting turns to heal. Velius can also summon Titan, use Loss (100 % Confusion) and Seal (100 % Petrify).