complete the 40 floor dungeon you unlock by defeating tiamat then complete it(although the boss at the end has more HP hes not as hard as omega and shinryu.) to get the ultima weapon. make sure the knights on 999hp because ultima weapon ignores defence and its attack powers based on the characters HP. when you fight omega and shinryu cast haste to make your knight do 16hits and keep him on as high health as you can and he can deal up to 2,000 damage( shinryu and omega both have 35,000 HP). seems as you sont have white or black mages to deal heavy damage id use the master and the ninja to use curative items (unless the monk is powerful enough to do a lot of damage to omega or shinryu. omega has much higher defence than shinryu though.) and only use your red wizard for healing and casting haste.